My situation:
I installed Neptune last year, version 8.1 I gather from /etc/os-release. I decided to upgrade to Debian testing/trixie to get newer versions of some programs I use quite often. There was not a Neptune trixie repository so I commented out /etc/apt/sources.list.d/neptune.list, which was the only Neptune-oriented repository that was not disabled by default, Then I switched my Debian bookworm sources.list repositories to trixie and upgraded and did a dist-upgrade, and for a few months was a very happy camper with my shiny Debian testing operating system. I didn't bother to go back a remove any Neptune packages, I believe. Things were great until yesterday when I did my regular "sudo apt full-upgrade" and rebooted and no longer had access to my mouse or keyboard! Ouch. I tried getting to a TTY with Ctrl+Alt+Fx, but no keyboard, no TTY!

I figured that the DU must have thrust me into wayland on KDE 6 and wanted to kill sddm and log in with X11, so I SSHed to my Neptune computer and stopped the sddm service, which did allow me to log in on the Neptune box directly, but then I realized that apt/aptitude were wedged shut and I couldn't add or upgrade any programs. The problem was some diversion set up by the neptune-base package, so I tried to remove neptune-base, but it wouldn't let me.

# apt remove neptune-base
REMOVING:                       
  neptune-base

Summary:
  Upgrading: 0, Installing: 0, Removing: 1, Not Upgrading: 1
  1 not fully installed or removed.
  Freed space: 61.6 MB

Continue? [Y/n] y
(Reading database ... 406175 files and directories currently installed.)
Removing neptune-base (4.3.27) ...
Generating grub configuration file ...
Found background image: resist.png
Found linux image: /boot/vmlinuz-6.6.15-amd64
Found initrd image: /boot/initrd.img-6.6.15-amd64
Found linux image: /boot/vmlinuz-6.6.13-amd64
Found initrd image: /boot/initrd.img-6.6.13-amd64
Found linux image: /boot/vmlinuz-6.1.0-3-amd64
Found initrd image: /boot/initrd.img-6.1.0-3-amd64
Adding boot menu entry for UEFI Firmware Settings ...
done
No diversion 'diversion of /usr/share/kde4/apps/plasma-desktop/init/00-defaultLayout.js to /usr/share/kde4/apps/plasma-desktop/init/00-defaultLayout.js.orig by neptune-base', none removed.
No diversion 'diversion of /usr/share/kde4/apps/plasma-desktop/updates/addShowActivitiesManagerPlasmoid.js to /usr/share/kde4/apps/plasma-desktop/updates/addShowActivitiesManagerPlasmoid.js.orig by neptune-base', none removed.
Removing 'diversion of /etc/os-release to /etc/os-release.orig by neptune-base'
dpkg-divert: error: rename involves overwriting '/etc/os-release' with
  different file '/etc/os-release.orig', not allowed
dpkg: error processing package neptune-base (--remove):
 installed neptune-base package post-removal script subprocess returned error exit status 2
dpkg: too many errors, stopping
Errors were encountered while processing:
 neptune-base
Processing was halted because there were too many errors.
Error: Sub-process /usr/bin/dpkg returned an error code (1)

So I reenabled the regular neptune.list bookworm repository. That seemed to unwedge apt to some extent, I could do a dist-upgrade, which reinstalled neptune-base and a subsequent DU would install libzvbi-common, both from the neptune repository. I was able to install one individual programs that had been removed (scribus), but mostly when I would attempt to install something to get a functioning system I would get an error message like this:

apt-get install audacious
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
audacious is already the newest version (4.3.1-2+b1).
The following packages will be REMOVED:
  neptune-base
0 upgraded, 0 newly installed, 1 to remove and 1 not upgraded.
1 not fully installed or removed.
After this operation, 61.6 MB disk space will be freed.
Do you want to continue? [Y/n] y
(Reading database ... 406175 files and directories currently installed.)
Removing neptune-base (4.3.27) ...
Generating grub configuration file ...
Found background image: resist.png
Found linux image: /boot/vmlinuz-6.6.15-amd64
Found initrd image: /boot/initrd.img-6.6.15-amd64
Found linux image: /boot/vmlinuz-6.6.13-amd64
Found initrd image: /boot/initrd.img-6.6.13-amd64
Found linux image: /boot/vmlinuz-6.1.0-3-amd64
Found initrd image: /boot/initrd.img-6.1.0-3-amd64
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot entries.
Found Debian GNU/Linux 12 (bookworm) on /dev/mapper/liva--vg-root
Adding boot menu entry for UEFI Firmware Settings ...
done
No diversion 'diversion of /usr/share/kde4/apps/plasma-desktop/init/00-defaultLayout.js to /usr/share/kde4/apps/plasma-desktop/init/00-defaultLayout.js.orig by neptune-base', none removed.
No diversion 'diversion of /usr/share/kde4/apps/plasma-desktop/updates/addShowActivitiesManagerPlasmoid.js to /usr/share/kde4/apps/plasma-desktop/updates/addShowActivitiesManagerPlasmoid.js.orig by neptune-base', none removed.
Removing 'diversion of /etc/os-release to /etc/os-release.orig by neptune-base'
dpkg-divert: error: rename involves overwriting '/etc/os-release' with
  different file '/etc/os-release.orig', not allowed
dpkg: error processing package neptune-base (--remove):
 installed neptune-base package post-removal script subprocess returned error exit status 2
dpkg: too many errors, stopping
Errors were encountered while processing:
 neptune-base
Processing was halted because there were too many errors.
E: Sub-process /usr/bin/dpkg returned an error code (1)

Enabling the neptune backports repository did not change the situation. Neither did replacing the Neptune /etc/os-release with a version from another vanilla Debian testing system. so I reverted to the original os-release file:

NAME="Neptune"
VERSION_ID="8.1"
VERSION="8.1 (Juna)"
ID=neptune
ID_LIKE=debian
HOME_URL="https://www.neptuneos.com/"
SUPPORT_URL="https://forum.neptuneos.com"
BUG_REPORT_URL="https://forum.neptuneos.com"
VERSION_CODENAME=bookworm
PRETTY_NAME="Neptune 8.1"

Can anyone help me un-Neptune my system so I can restore it to working condition? I'm sure if I can unstick apt/dpkg/etc I can fix my box. Neptune's customizations are nice enough but I need to be able to upgrade my system.

PS: I deleted my first version of this post to edit it a bit for clarity, and am reposting.