I don't know if I understand your problem right.
You think you can't log on as user because beryl autostarting twice gives you a whitescreen ? If so, why don't you just remove the autostart items out of the autostart folder as root ? Its /home/user/.kde/Autostart for kde and /home/user/.config/autostart for gnome I think (if you meant the session manager of gnome with "session list", you didn't tell what youre using).
I just wanted to say, that I had the white screen problem as well after installing beryl (completely white screen, did not try to rotate the cube). Unchecking the "sync to vblank" option fixed it for me. I'm using beryl with nvidia gl driver.
However, this whitescreen problem seems to have more than 1 reason, just look in the unofficial nvidia support forum (www.nvnews.net)
, there are a ton of posts concerning this problem.