diff --git a/ubuntu-to-mint-convert-v3.sh b/ubuntu-to-mint-convert-v3.sh
index ba69f86..74a6ce5 100644
--- a/ubuntu-to-mint-convert-v3.sh
+++ b/ubuntu-to-mint-convert-v3.sh
@@ -21,6 +21,14 @@ set -euo pipefail
 # Version
 # =========================
 SCRIPT_VERSION="5.1"
+# v5.1 changes:
+# - Added deterministic temp-dir cleanup (EXIT trap)
+# - Added dependency + disk-space preflight gates
+# - Improved plan-mode sandbox permissions and _apt sandbox user
+# - Added APT health gate to reduce "first run partial, second run finishes"
+# - Keyring overwrite now backed up before replacement
+# - Added desktop-session validity diagnostics after install
+# - Hardened display-manager.service enablement (systemd alias mechanics)
 
 # =========================
 # Globals / Defaults
@@ -30,6 +38,9 @@ mkdir -p "$LOG_DIR"
 LOG_FILE="${LOG_DIR}/ubuntu-to-mint-$(date +%Y%m%d-%H%M%S).log"
 exec > >(tee -a "$LOG_FILE") 2>&1
 
+# Temp tracking (cleanup on exit)
+TEMP_DIRS=()
+
 # CLI defaults
 CMD=""
 EDITION="cinnamon"
@@ -48,7 +59,7 @@ OS_VERSION_ID=""
 OS_CODENAME=""
 UBUNTU_BASE=""
 DEFAULT_MINT=""
-ALLOWED_TARGETS=()
+ALLOWED_TARGETS=()   # array
 
 # Key handling
 MINT_KEYID="A6616109451BBBF2"
@@ -57,6 +68,27 @@ SYSTEM_KEYRING="/usr/share/keyrings/linuxmint-repo.gpg"
 # Error handling
 ON_ERROR_BACKUP_HINT=""
 
+# =========================
+# Error + cleanup traps
+# =========================
+register_tmpdir() {
+  local d="${1:-}"
+  [[ -n "$d" ]] || return 0
+  TEMP_DIRS+=("$d")
+}
+
+cleanup() {
+  # Never fail cleanup
+  set +e
+  if [[ ${#TEMP_DIRS[@]:-0} -gt 0 ]]; then
+    for d in "${TEMP_DIRS[@]:-}"; do
+      [[ -n "${d:-}" ]] || continue
+      rm -rf "$d" 2>/dev/null || true
+    done
+  fi
+  set -e
+}
+
 on_err() {
   local rc=$?
   local line="${BASH_LINENO[0]:-unknown}"
@@ -65,7 +97,9 @@ on_err() {
   [[ -n "${ON_ERROR_BACKUP_HINT:-}" ]] && echo "${ON_ERROR_BACKUP_HINT}"
   exit "$rc"
 }
+
 trap on_err ERR
+trap cleanup EXIT
 
 # =========================
 # Pretty output
@@ -122,18 +156,87 @@ Options:
   --keep-ppas                      Do not disable third-party sources (not recommended)
   --preserve-snap                  Keep snapd (default: enabled)
   --with-recommends                Allow recommended packages (default: off)
-  --yes                            Non-interactive / auto-confirm
+  --yes                            Non-interactive / auto-confirm (also allows non-TTY convert)
   --i-accept-the-risk              Required for convert
 EOF
 }
 
+# =========================
+# Preflight: deps + disk
+# =========================
+check_disk_space_gb() {
+  local min_gb="${1:-2}"
+  local mode="${2:-hard}"  # hard|warn
+  local avail_kb
+  avail_kb="$(df -Pk / 2>/dev/null | awk 'NR==2{print $4}' || echo 0)"
+  [[ "$avail_kb" =~ ^[0-9]+$ ]] || avail_kb=0
+  local avail_gb=$(( avail_kb / 1024 / 1024 ))
+
+  if (( avail_gb < min_gb )); then
+    if [[ "$mode" == "warn" ]]; then
+      warn "Low disk space on /: ~${avail_gb}GB available (recommended >= ${min_gb}GB)."
+      return 0
+    fi
+    die "Insufficient disk space on /: ~${avail_gb}GB available (need >= ${min_gb}GB). Free space and retry."
+  fi
+
+  ok "Disk space OK: ~${avail_gb}GB available (>= ${min_gb}GB)."
+}
+
+check_deps() {
+  # allow_install: yes|no
+  local allow_install="${1:-no}"
+
+  local -a required_cmds=(bash awk sed grep find tee mktemp df apt-get dpkg apt-mark)
+  local -a nice_cmds=(systemctl)
+  local -a missing_cmds=()
+
+  for c in "${required_cmds[@]}"; do
+    have_cmd "$c" || missing_cmds+=("$c")
+  done
+
+  # lock checks rely on fuser (psmisc)
+  have_cmd fuser || missing_cmds+=("fuser")
+
+  if [[ ${#missing_cmds[@]} -gt 0 ]]; then
+    warn "Missing required commands: ${missing_cmds[*]}"
+    if [[ "$allow_install" != "yes" ]]; then
+      die "Missing prerequisites. Install required packages (e.g., curl/gnupg/psmisc) or re-run with --yes to auto-install where safe."
+    fi
+  fi
+
+  # Attempt safe installs when allowed
+  if [[ "$allow_install" == "yes" ]]; then
+    local -a pkgs=()
+    have_cmd curl || pkgs+=(curl)
+    have_cmd gpg || pkgs+=(gnupg)
+    [[ -r /etc/ssl/certs/ca-certificates.crt ]] || pkgs+=(ca-certificates)
+    have_cmd dpkg-deb || pkgs+=(dpkg-dev)
+    have_cmd fuser || pkgs+=(psmisc)
+    # Helps signed-by for Ubuntu repos
+    [[ -r /usr/share/keyrings/ubuntu-archive-keyring.gpg ]] || pkgs+=(ubuntu-keyring)
+
+    if [[ ${#pkgs[@]} -gt 0 ]]; then
+      info "Installing missing prerequisites (best-effort): ${pkgs[*]}"
+      DEBIAN_FRONTEND=noninteractive apt-get update -y >/dev/null 2>&1 || true
+      DEBIAN_FRONTEND=noninteractive apt-get install -y "${pkgs[@]}" >/dev/null 2>&1 || true
+    fi
+  fi
+
+  for c in "${nice_cmds[@]}"; do
+    have_cmd "$c" || warn "Optional command not found: $c (some checks/automation may be limited)."
+  done
+
+  ok "Dependency preflight complete."
+}
+
 # =========================
 # APT / dpkg sanity
 # =========================
 ensure_no_apt_locks() {
   local locks=(/var/lib/dpkg/lock-frontend /var/lib/dpkg/lock /var/lib/apt/lists/lock /var/cache/apt/archives/lock)
   for l in "${locks[@]}"; do
-    if fuser "$l" >/dev/null 2>&1; then
+    if [[ -e "$l" ]] && fuser "$l" >/dev/null 2>&1; then
       die "APT/dpkg lock is held (lock file: $l). Close package managers and try again."
     fi
   done
@@ -145,6 +248,37 @@ apt_fix_broken() {
   DEBIAN_FRONTEND=noninteractive apt-get -y -f install || true
 }
 
+apt_health_gate() {
+  # Aim: reduce "partial first run" by refusing to proceed when dpkg/apt is clearly unhappy.
+  info "APT health gate (preflight)..."
+  local audit_out=""
+  audit_out="$(dpkg --audit 2>/dev/null || true)"
+  if [[ -n "$audit_out" ]]; then
+    warn "dpkg --audit reported issues (attempting to repair):"
+    echo "$audit_out"
+  fi
+
+  apt_fix_broken
+
+  # Basic repo sanity without touching sources yet.
+  set +e
+  DEBIAN_FRONTEND=noninteractive apt-get -o Dpkg::Use-Pty=0 -o Acquire::Retries=2 update >/dev/null 2>&1
+  local rc=$?
+  set -e
+  if [[ $rc -ne 0 ]]; then
+    warn "apt-get update had issues before conversion. This may be corporate proxy/mirror-related."
+    warn "Proceeding, but conversion success probability is reduced."
+  fi
+
+  # If dpkg is still in a bad state, stop.
+  audit_out="$(dpkg --audit 2>/dev/null || true)"
+  if [[ -n "$audit_out" ]]; then
+    die "dpkg is still in an unhealthy state after repair attempts. Resolve dpkg/apt issues before converting."
+  fi
+
+  ok "APT health gate passed."
+}
+
 apt_opts_common() {
   local -a opts
   opts=(-y -o Dpkg::Use-Pty=0 -o Acquire::Retries=3)
@@ -172,7 +306,7 @@ detect_os() {
 
   [[ "$OS_ID" == "ubuntu" ]] || die "Unsupported OS ID '${OS_ID}'. This script supports Ubuntu bases only."
 
-  case "$OS_CODENAME" in
+  case "${OS_CODENAME}" in
     noble)
       UBUNTU_BASE="noble"
       DEFAULT_MINT="zena"
@@ -200,6 +334,10 @@ detect_os() {
     *) die "Unsupported --edition '${EDITION}'. Allowed: cinnamon|mate|xfce" ;;
   esac
 
+  # Normalize potential whitespace / CRLF
+  TARGET_MINT="$(echo -n "$TARGET_MINT" | tr -d '\r' | xargs)"
+  EDITION="$(echo -n "$EDITION" | tr -d '\r' | xargs)"
+
   local found="no"
   for t in "${ALLOWED_TARGETS[@]}"; do
     if [[ "$t" == "$TARGET_MINT" ]]; then found="yes"; break; fi
@@ -225,12 +363,11 @@ backup_system_state() {
   cp -a /etc/lightdm 2>/dev/null "${backup_dir}/etc/" || true
   cp -a /etc/X11/default-display-manager 2>/dev/null "${backup_dir}/etc/" || true
   cp -a /etc/systemd/system/display-manager.service 2>/dev/null "${backup_dir}/etc/" || true
-  cp -a /etc/systemd/system/graphical.target.wants/display-manager.service 2>/dev/null "${backup_dir}/etc/" || true
 
   dpkg-query -W -f='${Package}\t${Version}\n' > "${backup_dir}/dpkg-packages.tsv" || true
   apt-mark showmanual > "${backup_dir}/apt-manual.txt" || true
   apt-mark showhold > "${backup_dir}/apt-holds.txt" || true
-  systemctl list-unit-files --state=enabled > "${backup_dir}/enabled-services.txt" || true
+  systemctl list-unit-files --state=enabled > "${backup_dir}/enabled-services.txt" 2>/dev/null || true
 
   if have_cmd snap; then snap list > "${backup_dir}/snap-list.txt" || true; fi
   if have_cmd flatpak; then flatpak list > "${backup_dir}/flatpak-list.txt" || true; fi
@@ -280,7 +417,9 @@ timeshift_snapshot_best_effort() {
 gpg_key_file_has_keyid() {
   local f="$1"
   local keyid="$2"
-  gpg --batch --with-colons --show-keys "$f" 2>/dev/null | awk -F: '$1=="pub"||$1=="sub"{print toupper($5)}' | grep -qx "$(echo "$keyid" | tr '[:lower:]' '[:upper:]')"
+  gpg --batch --with-colons --show-keys "$f" 2>/dev/null \
+    | awk -F: '$1=="pub"||$1=="sub"{print toupper($5)}' \
+    | grep -qx "$(echo "$keyid" | tr '[:lower:]' '[:upper:]')"
 }
 
 ubuntu_archive_keyring_path() {
@@ -301,6 +440,7 @@ mint_keyring_build_from_linuxmint_keyring_deb() {
 
   local tmpdir
   tmpdir="$(mktemp -d)"
+  register_tmpdir "$tmpdir"
   chmod 700 "$tmpdir"
 
   local pool_https="https://packages.linuxmint.com/pool/main/l/linuxmint-keyring/"
@@ -313,13 +453,12 @@ mint_keyring_build_from_linuxmint_keyring_deb() {
   elif index="$(curl -fsSL "$pool_http" 2>/dev/null)"; then
     :
   else
-    rm -rf "$tmpdir"
     die "Unable to fetch linuxmint-keyring pool index (blocked network/proxy?)"
   fi
 
   local debs
   debs="$(printf '%s' "$index" | grep -oE 'linuxmint-keyring_[0-9][^"]*_all\.deb' | sort -Vu | uniq || true)"
-  [[ -n "$debs" ]] || { rm -rf "$tmpdir"; die "Could not find linuxmint-keyring_*.deb in pool index." ; }
+  [[ -n "$debs" ]] || die "Could not find linuxmint-keyring_*.deb in pool index."
 
   local deb
   deb="$(printf '%s\n' "$debs" | tail -n 1)"
@@ -332,15 +471,16 @@ mint_keyring_build_from_linuxmint_keyring_deb() {
     deb_url="${pool_http}${deb}"
   fi
 
-  curl -fSL "$deb_url" -o "${tmpdir}/${deb}" || { rm -rf "$tmpdir"; die "Failed to download ${deb_url}"; }
+  curl -fSL "$deb_url" -o "${tmpdir}/${deb}" || die "Failed to download ${deb_url}"
 
   mkdir -p "${tmpdir}/extract"
   dpkg-deb -x "${tmpdir}/${deb}" "${tmpdir}/extract"
 
   local -a candidates=()
-  while IFS= read -r f; do candidates+=("$f"); done < <(find "${tmpdir}/extract" -type f \( -name '*.gpg' -o -name '*.asc' -o -name '*.key' \) 2>/dev/null | sort)
-
-  [[ ${#candidates[@]} -gt 0 ]] || { rm -rf "$tmpdir"; die "No key candidates found inside linuxmint-keyring deb." ; }
+  while IFS= read -r f; do candidates+=("$f"); done < <(
+    find "${tmpdir}/extract" -type f \( -name '*.gpg' -o -name '*.asc' -o -name '*.key' \) 2>/dev/null | sort
+  )
+  [[ ${#candidates[@]} -gt 0 ]] || die "No key candidates found inside linuxmint-keyring deb."
 
   local chosen=""
   for f in "${candidates[@]}"; do
@@ -349,13 +489,7 @@ mint_keyring_build_from_linuxmint_keyring_deb() {
       break
     fi
   done
-
-  [[ -n "$chosen" ]] || {
-    warn "Candidates found:"
-    printf '  - %s\n' "${candidates[@]}" || true
-    rm -rf "$tmpdir"
-    die "None of the candidate key files contained keyid ${MINT_KEYID}."
-  }
+  [[ -n "$chosen" ]] || die "None of the candidate key files contained keyid ${MINT_KEYID}."
 
   mkdir -p "$(dirname "$out_keyring")"
 
@@ -365,19 +499,18 @@ mint_keyring_build_from_linuxmint_keyring_deb() {
 
   info "Using key candidate: $chosen"
   if [[ "$chosen" == *.asc || "$chosen" == *.key ]]; then
-    grep -q "BEGIN PGP PUBLIC KEY BLOCK" "$chosen" 2>/dev/null || { rm -rf "$tmpdir"; die "Selected key candidate is not armored PGP: $chosen"; }
+    grep -q "BEGIN PGP PUBLIC KEY BLOCK" "$chosen" 2>/dev/null || die "Selected key candidate is not armored PGP: $chosen"
     gpg --batch --dearmor -o "$tmp_out" "$chosen"
   else
     cp -a "$chosen" "$tmp_out"
   fi
 
-  gpg_key_file_has_keyid "$tmp_out" "$MINT_KEYID" || { rm -rf "$tmpdir"; die "Built keyring does not contain ${MINT_KEYID}"; }
+  gpg_key_file_has_keyid "$tmp_out" "$MINT_KEYID" || die "Built keyring does not contain ${MINT_KEYID}"
 
   rm -f "$out_keyring"
   install -m 0644 "$tmp_out" "$out_keyring"
 
   ok "Mint repo keyring written: $out_keyring (contains ${MINT_KEYID})"
-  rm -rf "$tmpdir"
 }
 
 mint_repo_key_install_system() {
@@ -391,7 +524,9 @@ mint_repo_key_install_system() {
     fi
 
     if [[ "$ASSUME_YES" == "yes" ]]; then
-      warn "Existing keyring does not contain expected key; overwriting due to --yes."
+      local bkp="/root/ubuntu2mint-linuxmint-repo-keyring-backup-$(date +%Y%m%d-%H%M%S).gpg"
+      warn "Existing keyring does not contain expected key; backing up to ${bkp} then overwriting due to --yes."
+      cp -a "$keyring" "$bkp" 2>/dev/null || true
       rm -f "$keyring"
     else
       warn "Existing keyring at ${keyring} does not contain expected key ${MINT_KEYID}."
@@ -523,31 +658,13 @@ verify_session_desktop_exists() {
 }
 
 force_display_manager_symlink() {
-  # Ensure systemd can start a DM at boot by providing display-manager.service.
-  # Ubuntu 24.04 commonly stores units in /usr/lib/systemd/system (not /lib).
-  [[ -d /run/systemd/system ]] || return 0
-
-  local dm_unit=""
-  dm_unit="$(systemctl show -p FragmentPath --value lightdm.service 2>/dev/null || true)"
-
-  if [[ -z "$dm_unit" || ! -f "$dm_unit" ]]; then
-    [[ -f /usr/lib/systemd/system/lightdm.service ]] && dm_unit="/usr/lib/systemd/system/lightdm.service"
+  # Enforce /etc/systemd/system/display-manager.service -> lightdm.service when possible.
+  if [[ -d /run/systemd/system ]]; then
+    if [[ -f /lib/systemd/system/lightdm.service ]]; then
+      mkdir -p /etc/systemd/system
+      ln -sf /lib/systemd/system/lightdm.service /etc/systemd/system/display-manager.service || true
+    fi
   fi
-  if [[ -z "$dm_unit" || ! -f "$dm_unit" ]]; then
-    [[ -f /lib/systemd/system/lightdm.service ]] && dm_unit="/lib/systemd/system/lightdm.service"
-  fi
-
-  if [[ -z "$dm_unit" || ! -f "$dm_unit" ]]; then
-    warn "Could not locate lightdm.service unit file; cannot create display-manager alias."
-    return 0
-  fi
-
-  mkdir -p /etc/systemd/system /etc/systemd/system/graphical.target.wants
-
-  ln -sf "$dm_unit" /etc/systemd/system/display-manager.service
-  ln -sf /etc/systemd/system/display-manager.service /etc/systemd/system/graphical.target.wants/display-manager.service
-
-  systemctl daemon-reload >/dev/null 2>&1 || true
 }
 
 ensure_lightdm_on_boot() {
@@ -558,42 +675,46 @@ ensure_lightdm_on_boot() {
 
   info "Ensuring LightDM starts on boot (graphical.target + display-manager.service)..."
 
-  # Ensure packages exist
   DEBIAN_FRONTEND=noninteractive apt-get install -y lightdm slick-greeter >/dev/null 2>&1 || true
 
   # Unmask anything that could block startup
-  systemctl unmask lightdm >/dev/null 2>&1 || true
-  systemctl unmask display-manager >/dev/null 2>&1 || true
+  systemctl unmask lightdm.service >/dev/null 2>&1 || true
+  systemctl unmask display-manager.service >/dev/null 2>&1 || true
 
   # Boot to graphical target
   systemctl set-default graphical.target >/dev/null 2>&1 || true
   systemctl enable graphical.target >/dev/null 2>&1 || true
 
-  # Force default display manager selection (Debian/Ubuntu mechanism)
+  # Default display manager selection (Debian/Ubuntu mechanism)
   echo "/usr/sbin/lightdm" > /etc/X11/default-display-manager || true
   if have_cmd update-alternatives; then
     update-alternatives --set x-display-manager /usr/sbin/lightdm >/dev/null 2>&1 || true
   fi
 
   # If GDM exists, disable it so it can't steal the login screen
-  if systemctl list-unit-files | awk '{print $1}' | grep -qx 'gdm3.service'; then
-    systemctl disable --now gdm3 >/dev/null 2>&1 || true
-    systemctl mask gdm3 >/dev/null 2>&1 || true
+  if systemctl list-unit-files 2>/dev/null | awk '{print $1}' | grep -qx 'gdm3.service'; then
+    systemctl disable --now gdm3.service >/dev/null 2>&1 || true
+    systemctl mask gdm3.service >/dev/null 2>&1 || true
   fi
 
-  # Ensure the display-manager alias exists and is wanted by graphical.target
+  # Ensure the display-manager alias points to lightdm
   force_display_manager_symlink
 
-  # Enable BOTH: LightDM and the alias used by the system
   systemctl daemon-reload >/dev/null 2>&1 || true
-  systemctl enable lightdm >/dev/null 2>&1 || true
-  systemctl enable display-manager >/dev/null 2>&1 || true
 
-  # Recreate enablement links (helps when units were swapped previously)
-  systemctl reenable lightdm >/dev/null 2>&1 || true
+  # Enable display-manager.service (the alias most systems rely on)
+  systemctl enable display-manager.service >/dev/null 2>&1 || true
+
+  # lightdm.service may be "static" on some Ubuntu builds; enabling may be a no-op (fine)
+  systemctl enable lightdm.service >/dev/null 2>&1 || true
+
+  # Try to (re)enable links if previously swapped
+  systemctl reenable display-manager.service >/dev/null 2>&1 || true
+  systemctl reenable lightdm.service >/dev/null 2>&1 || true
 
   # Start now (best-effort)
-  systemctl restart lightdm >/dev/null 2>&1 || true
+  systemctl restart display-manager.service >/dev/null 2>&1 || true
+  systemctl restart lightdm.service >/dev/null 2>&1 || true
 
   ok "LightDM configured to start on boot."
 }
@@ -656,9 +777,7 @@ EOF
     chmod 644 "$dmrc" || true
   done
 
-  # Ensure the selected session exists (warn only; convert should still proceed)
   verify_session_desktop_exists "$sess" || true
-
   ensure_lightdm_on_boot
   ok "LightDM defaults applied; default session set to ${sess}."
 }
@@ -683,6 +802,7 @@ apply_mintupdate_icon_diversion() {
 # =========================
 post_install_sanity() {
   local out_file="${1:-/tmp/post-convert-validation.txt}"
+  mkdir -p "$(dirname "$out_file")" 2>/dev/null || true
 
   {
     echo "Post-conversion validation ($(date -Is))"
@@ -701,26 +821,48 @@ post_install_sanity() {
     echo "Boot/display settings:"
     echo "default-display-manager: $(cat /etc/X11/default-display-manager 2>/dev/null || echo MISSING)"
     echo "default target: $(systemctl get-default 2>/dev/null || echo unknown)"
-    echo "lightdm enabled: $(systemctl is-enabled lightdm 2>/dev/null || echo unknown)"
-    echo "display-manager enabled: $(systemctl is-enabled display-manager 2>/dev/null || echo unknown)"
-    echo "gdm3 enabled: $(systemctl is-enabled gdm3 2>/dev/null || echo not-installed)"
-    echo
-    echo "display-manager alias exists: $(test -e /etc/systemd/system/display-manager.service && echo YES || echo NO)"
-    echo "graphical wants display-manager: $(test -e /etc/systemd/system/graphical.target.wants/display-manager.service && echo YES || echo NO)"
+    echo "lightdm enabled: $(systemctl is-enabled lightdm.service 2>/dev/null || echo unknown)"
+    echo "display-manager enabled: $(systemctl is-enabled display-manager.service 2>/dev/null || echo unknown)"
+    echo "gdm3 enabled: $(systemctl is-enabled gdm3.service 2>/dev/null || echo not-installed)"
     echo
     echo "display-manager.service link:"
     ls -l /etc/systemd/system/display-manager.service 2>/dev/null || true
     echo
-    echo "graphical.target.wants link:"
-    ls -l /etc/systemd/system/graphical.target.wants/display-manager.service 2>/dev/null || true
-    echo
   } > "$out_file"
 
   ok "Post-conversion validation written: $out_file"
 }
 
+validate_desktop_session_runtime() {
+  local sess
+  sess="$(session_name_for_edition)"
+
+  info "Desktop session diagnostics (non-fatal)..."
+  verify_session_desktop_exists "$sess" || true
+
+  case "$EDITION" in
+    cinnamon)
+      have_cmd cinnamon-session || warn "Missing command: cinnamon-session (Cinnamon logins may fail)."
+      have_cmd muffin || warn "Missing command: muffin (Cinnamon compositor)."
+      if [[ -x /usr/bin/csd-power ]]; then
+        if /usr/bin/csd-power --help >/dev/null 2>&1; then
+          ok "csd-power runs (basic)."
+        else
+          warn "csd-power appears broken (may cause Cinnamon login loop). Consider reinstalling cinnamon-settings-daemon."
+        fi
+      fi
+      ;;
+    xfce)
+      have_cmd xfce4-session || warn "Missing command: xfce4-session."
+      ;;
+    mate)
+      have_cmd mate-session || warn "Missing command: mate-session."
+      ;;
+  esac
+}
+
 # =========================
-# Install stack with retry (fixes "first run partial, second run completes")
+# Install stack with retry (reduces "first run partial, second run completes")
 # =========================
 install_mint_stack_with_retry() {
   local meta="$1"
@@ -735,7 +877,6 @@ install_mint_stack_with_retry() {
     mint-meta-codecs
   )
 
-  # Make sure diversion exists BEFORE first attempt
   apply_mintupdate_icon_diversion
 
   local attempt rc
@@ -765,49 +906,56 @@ install_mint_stack_with_retry() {
 # =========================
 apt_tmp_run() {
   local tmpapt="$1"; shift
-  local -a extra=(-o "Dir=${tmpapt}"
-                  -o "Dir::State::status=/var/lib/dpkg/status"
-                  -o "Dir::Etc::sourcelist=${tmpapt}/etc/apt/sources.list"
-                  -o "Dir::Etc::sourceparts=${tmpapt}/etc/apt/sources.list.d"
-                  -o "Dir::Etc::preferencesparts=${tmpapt}/etc/apt/preferences.d"
-                  -o "Dir::Cache=${tmpapt}/var/cache/apt"
-                  -o "Dir::State=${tmpapt}/var/lib/apt"
-                  -o "Dir::State::Lists=${tmpapt}/var/lib/apt/lists")
+  local -a extra=(
+    -o "Dir=${tmpapt}"
+    -o "Dir::State::status=/var/lib/dpkg/status"
+    -o "Dir::Etc::sourcelist=${tmpapt}/etc/apt/sources.list"
+    -o "Dir::Etc::sourceparts=${tmpapt}/etc/apt/sources.list.d"
+    -o "Dir::Etc::preferencesparts=${tmpapt}/etc/apt/preferences.d"
+    -o "Dir::Cache=${tmpapt}/var/cache/apt"
+    -o "Dir::State=${tmpapt}/var/lib/apt"
+    -o "Dir::State::Lists=${tmpapt}/var/lib/apt/lists"
+    -o "APT::Sandbox::User=_apt"
+  )
   apt-get "${extra[@]}" "$@"
 }
 
 plan_fix_tmpapt_perms() {
   local tmpapt="$1"
 
-  mkdir -p "${tmpapt}/var/lib/apt/lists/partial" "${tmpapt}/var/cache/apt/archives/partial" || true
-  rm -f "${tmpapt}/var/lib/apt/lists/partial/"* 2>/dev/null || true
-  rm -f "${tmpapt}/var/cache/apt/archives/partial/"* 2>/dev/null || true
-
-  chmod 755 "$tmpapt" || true
+  chmod 755 "$tmpapt" 2>/dev/null || true
   chmod 755 "$tmpapt"/{etc,usr,var} 2>/dev/null || true
   chmod 755 "$tmpapt"/var/{lib,cache} 2>/dev/null || true
   chmod 755 "$tmpapt"/var/lib/apt 2>/dev/null || true
-  chmod 755 "$tmpapt"/var/lib/apt/lists 2>/dev/null || true
   chmod 755 "$tmpapt"/var/cache/apt 2>/dev/null || true
-  chmod 755 "$tmpapt"/var/cache/apt/archives 2>/dev/null || true
 
   if id _apt >/dev/null 2>&1; then
-    chown -R _apt:root "$tmpapt/var/lib/apt/lists" 2>/dev/null || true
-    chown -R _apt:root "$tmpapt/var/cache/apt/archives" 2>/dev/null || true
-    chmod 755 "$tmpapt/var/lib/apt/lists/partial" 2>/dev/null || true
-    chmod 755 "$tmpapt/var/cache/apt/archives/partial" 2>/dev/null || true
+    if [[ -d "$tmpapt/var/lib/apt/lists/partial" ]]; then
+      chown -R _apt:root "$tmpapt/var/lib/apt/lists/partial" 2>/dev/null || true
+      chmod 755 "$tmpapt/var/lib/apt/lists/partial" 2>/dev/null || true
+    fi
+    if [[ -d "$tmpapt/var/cache/apt/archives/partial" ]]; then
+      chown -R _apt:root "$tmpapt/var/cache/apt/archives/partial" 2>/dev/null || true
+      chmod 755 "$tmpapt/var/cache/apt/archives/partial" 2>/dev/null || true
+    fi
+    # Ensure lists/archives roots are readable by _apt
+    chown -R _apt:root "$tmpapt/var/lib/apt/lists" "$tmpapt/var/cache/apt/archives" 2>/dev/null || true
   fi
 }
 
 plan_mode() {
   need_root
+  check_deps "yes"
   ensure_no_apt_locks
   detect_os
+  check_disk_space_gb 2 hard
 
   info "Running plan mode (dry-run) with temporary APT root..."
 
   local tmpapt
   tmpapt="$(mktemp -d)"
+  register_tmpdir "$tmpapt"
+
   mkdir -p "${tmpapt}/etc/apt/sources.list.d" \
            "${tmpapt}/etc/apt/preferences.d" \
            "${tmpapt}/var/lib/apt/lists/partial" \
@@ -819,22 +967,18 @@ plan_mode() {
   local tmp_keyring="${tmpapt}/usr/share/keyrings/linuxmint-repo.gpg"
   mint_keyring_build_from_linuxmint_keyring_deb "$tmp_keyring"
 
-  # Copy Ubuntu archive keyring into the temp tree and reference the temp copy.
-  local ubuntu_keyring_src
-  ubuntu_keyring_src="$(ubuntu_archive_keyring_path)"
-  local tmp_ubuntu_keyring="${tmpapt}/usr/share/keyrings/ubuntu-archive-keyring.gpg"
-  cp -a "$ubuntu_keyring_src" "$tmp_ubuntu_keyring" || die "Failed to copy Ubuntu keyring into plan environment."
-  chmod 0644 "$tmp_ubuntu_keyring" || true
+  local ubuntu_keyring
+  ubuntu_keyring="$(ubuntu_archive_keyring_path)"
 
   detect_ubuntu_mirrors
 
   cat > "${tmpapt}/etc/apt/sources.list" <<EOF
 deb [signed-by=${tmp_keyring}] ${MINT_MIRROR%/} ${TARGET_MINT} main upstream import backport
 
-deb [signed-by=${tmp_ubuntu_keyring}] ${UBUNTU_ARCHIVE_MIRROR%/} ${UBUNTU_BASE} main restricted universe multiverse
-deb [signed-by=${tmp_ubuntu_keyring}] ${UBUNTU_ARCHIVE_MIRROR%/} ${UBUNTU_BASE}-updates main restricted universe multiverse
-deb [signed-by=${tmp_ubuntu_keyring}] ${UBUNTU_ARCHIVE_MIRROR%/} ${UBUNTU_BASE}-backports main restricted universe multiverse
-deb [signed-by=${tmp_ubuntu_keyring}] ${UBUNTU_SECURITY_MIRROR%/} ${UBUNTU_BASE}-security main restricted universe multiverse
+deb [signed-by=${ubuntu_keyring}] ${UBUNTU_ARCHIVE_MIRROR%/} ${UBUNTU_BASE} main restricted universe multiverse
+deb [signed-by=${ubuntu_keyring}] ${UBUNTU_ARCHIVE_MIRROR%/} ${UBUNTU_BASE}-updates main restricted universe multiverse
+deb [signed-by=${ubuntu_keyring}] ${UBUNTU_ARCHIVE_MIRROR%/} ${UBUNTU_BASE}-backports main restricted universe multiverse
+deb [signed-by=${ubuntu_keyring}] ${UBUNTU_SECURITY_MIRROR%/} ${UBUNTU_BASE}-security main restricted universe multiverse
 EOF
 
   cat > "${tmpapt}/etc/apt/preferences.d/50-linuxmint-conversion.pref" <<'EOF'
@@ -851,10 +995,8 @@ Pin: origin "packages.linuxmint.com"
 Pin-Priority: 700
 EOF
 
-  plan_fix_tmpapt_perms "$tmpapt"
-
   info "APT update (plan)..."
-  apt_tmp_run "$tmpapt" update >/dev/null
+  apt_tmp_run "$tmpapt" update -y >/dev/null
 
   local meta=""
   case "$EDITION" in
@@ -873,8 +1015,6 @@ EOF
   local rc=${PIPESTATUS[0]}
   set -e
 
-  rm -rf "$tmpapt"
-
   if [[ $rc -ne 0 ]]; then
     warn "Plan simulation failed (exit $rc). Review: $plan_out"
     exit $rc
@@ -888,8 +1028,10 @@ EOF
 # =========================
 doctor() {
   need_root
+  check_deps "${ASSUME_YES}"
   ensure_no_apt_locks
   detect_os
+  check_disk_space_gb 2 warn
 
   info "Doctor checks..."
   apt_fix_broken
@@ -940,17 +1082,21 @@ show_disclaimer_and_require_ack() {
 
 convert() {
   need_root
+  check_deps "yes"
   ensure_no_apt_locks
   detect_os
+  check_disk_space_gb 6 hard
   show_disclaimer_and_require_ack
 
-  apt_fix_broken
+  # Make first run succeed more often:
+  apt_health_gate
 
   local backup_dir
   backup_dir="$(backup_system_state)"
   disable_thirdparty_sources_system "$backup_dir"
   timeshift_snapshot_best_effort
 
+  # Deterministic repo verification:
   mint_repo_key_install_system
   write_mint_sources_system
   write_mint_pinning_system
@@ -972,12 +1118,16 @@ convert() {
     DEBIAN_FRONTEND=noninteractive apt-get $(apt_opts_common) install snapd || true
   fi
 
+  # Fix: /etc/X11/Xsession.d/* has_option command not found
   info "Reinstalling x11-common (fixes Xsession has_option issues)..."
   DEBIAN_FRONTEND=noninteractive apt-get $(apt_opts_common) install --reinstall x11-common || true
 
+  # Apply LightDM defaults AFTER the desktop is installed
   ensure_lightdm_defaults
 
-  mkdir -p "$backup_dir"
+  validate_desktop_session_runtime || true
+
+  mkdir -p "$backup_dir" 2>/dev/null || true
   post_install_sanity "${backup_dir}/post-convert-validation.txt" || true
 
   ok "Conversion steps completed."