Ok, this is going to be trickier…
One solution I see would be to log in as root in a tty (ctrl+alt+F2 before logging in into your user, ctrl+alt+F1 to come back to X display), but if you didn’t put a password on root (default is no password), you will have to put a password first to root user.
However, I wonder if changing the username will work, Qubes OS may have your username somewhere in the configuration, if it’s using the actual username and not the uid, that will break stuff. But I understand it’s almost a test system you have right?