apt policy : These are Ubuntu phased updates: https://people.canonical.com/~ubuntu-archive/phased-updates.html
Theme changes are OK. Xfce uses GTK titlebar for CSD applications (these change when you change the "appearance" theme), but Xfwm for SSD applications (like Xed, these change when you change the theme in "Window Manager -> Style").
Regarding "#028: Themes" -- yes, changing from Mint-Y to Mint-X doesn't change the window titlebar theme. It still uses Mint-Y, even after logging out and back in.
Regarding "#110 Screensaver - lock screen - switch user " -- there is indeed not a switch user button on Xfce. If there another user account is created, it does show on the lock screen. So user switching is supported.
#043: apt policy: This items needs further review. The number of pinned packaged did not seem reasonable. Please see the following list at https://dpaste.com/HJW4DCWX7
#110 Screensaver - lock screen - switch user : did not see "The lock screen contains a "switch user" button"
#028: Themes: Themes do not see to be consistent when changed. When going to Mint-X some window elements stay as Mint-Y. I do not spend much time in the Xfce desktop. If someone else could take a look and confirm there is an issue with the theme changes.
Update on #039: did the triple boot process all over again. Issue only occurs when Xfce is installed after MATE, the MATE label goes away for the GRUB boot entry. With a dual boot of MATE and Cinnamon GRUB does not show a label as "Cinnamon", it is just Linux Mint 21 Vanessa (21) and Linux Mint 21 Vanessa (21) MATE.
#019: LSB and info file -- /etc/upstream-release/lsb-release, still shows info for Focal Fossa.
#039: Other OS in Grub -- please see the image at: https://justpaste.it/64izk
GRUB entries do not distinguish between MATE and Cinnamon. I just know that Cinnamon is in /dev/sda2 and MATE is in /dev/sda3
#000: Integrity check : SHA 256 check passed but there is no GRUB menu item to "Check the integrity of the medium" Noticed systemd: Failed to start casper-md5check Verify Live ISO checksums.