I use KDE on Debian. I have not encountered this, nor can I think of a reason why showkey would break a user's desktop session.

If the GUI login screen is still visible when it hangs, I suppose sddm might be having trouble. To investigate, I would run journalctl -f in a text console, and maybe tail -F /var/log/Xorg.0.log* in another, while attempting a GUI login. When it hangs, I would switch back to the text consoles and see if the most recent log messages hint at what's hanging.

*(Or whichever log file corresponds to the new X session, assuming you're using Xorg instead of Wayland.)

Could the fingerprint reader be causing the problem on the main account?

It sounds like you messed up the key binding for the enter key and fingerprint scanner. Boot to a live CD, mount your partition, chroot into the OS, delete your changes to whatever file that process you ran uses. You can try just deleting the file and reinstalling defaults if you can find a command that reinitializes it!.

If startx and everything else works fine, my guess is that sddm or whatever login manager you're using is messed up. Maybe reinstall/reset its configuration files and go from there.