How to enable fast user switching when Login Options are disabled?
This isn't a real fix, but I finally found a workaround. Even when fast user switching is disabled, you can still invoke this feature from the command line by running:

"/System/Library/CoreServices/Menu Extras/User.menu/Contents/Resources/CGSession" -suspend


This leaves your session running and goes back to the login screen.

I created an alias for this in my .bashrc, good enough for now.