Can't get past display manager

[Robbo]

Hi again, I guess I need some help working out what's gone wrong/what I've done this time.

The Problem:
When I boot up MX I get as far as lightDM. Normally my username would already be displayed as the default and I would just put in my password and be on my way. But my username is not there; so I input both my username and password and then get an error: "The session failed to start." If I try putting in root and the root password the login dialogue and panel disappear and then nothing happens and I need to shutdown manually.

The Lead Up:
I honestly keep my laptop on most of the time so it's probably been over a week since I last rebooted (and 3-4 days since I last logged out). All I can remember for sure that I've changed is what I did yesterday, so I'll list that here:
Fully updated and upgraded
Uninstalled Zoom and then reinstalled Zoom (using MX Package Manager)
Uninstalled Python3 and then reinstalled Python3 (using MX Package Manager)
Downloaded this lightsOn.sh script and moved it to my Home directory, in an attempt to stop xscreensaver from running while I'm watching videos on firefox: https://github.com/iye/lightsOn/blob/master/lightsOn.sh
Used Job Scheduler to run lightsOn.sh at reboot: @reboot ~/lightsOn.sh 590 &
Then I rebooted and the problem started.

What I've tried so far:
Switched to a Virtual Console to login with my username and password, but was given an error that it could not run /usr/bin/fish as that file did not exist (I had set fish as my default shell) and was promptly kicked out.
Restarted using a Live USB and confirmed that the root directory on my laptop was missing /usr/bin/fish (fish was working the last time I had used a terminal before this mess started). /usr/bin/fish was also missing from the list of shells in /etc/shells. So, I opened up /etc/passwd and changed the default shell for my user and root to be /usr/bin/bash.
Logged out of the live environment and booted up my system. Switched to the Virtual Console again and was able to login. Used crontab -e to remove the @reboot cronjob.
Rebooted, and still have the same problem.
So now I'm at a loss and looking for some wisdom and help.

[dolphin_oracle]

probably good to check the permission on the home folder and it contents. particularly the .Xauthority file.

might also be worth checking apt-history to see if fish was removed and when/why. could be two separate problems, but maybe not.

[fehlix]

Uninstalled Python3 and then reinstalled Python3 (using MX Package Manager)

Did you realy uninstalled "python3"
Uninstalling python3 - would remove a huge number of important packages including some needed for the login display manager. I guess if you really have done this, your system will no longer work properly.

[dolphin_oracle]

Uninstalled Python3 and then reinstalled Python3 (using MX Package Manager)

Did you realy uninstalled "python3"
Uninstalling python3 - would remove a huge number of important packages including some needed for the login display manager. I guess if you really have done this, your system will no longer work properly.

oh very good eye @fehlix . removing python3 would also remove fish.

[Robbo]

Did you realy uninstalled "python3"
Uninstalling python3 - would remove a huge number of important packages including some needed for the login display manager. I guess if you really have done this, your system will no longer work properly.

Well, I suppose the bagoverhead emoji in my original post was more relevant than I thought.

Yes, that is exactly what I did.
Thank you for the information.

Anything to do beyond a fresh install?

Am I okay to leave my home directory and just reinstall root?