I also agree that you need to get out of that room. It doesn't sound like this girl is someone who would be reasonable if you suggested arbitration Plus if she was already telling lies about you, what's stopping her from pulling something there too? Hopefully your other roommates haven't been influenced too much by this psycho, but if you try to have her removed, there might be problems with them if they are already on her side. I think your best option would be to see if there's any openings that you and your friend could take anywhere on campus. If you could room together again, great, but I'm sure you'll still be close friends if you end up living apart. Plus less stress makes for happier people.
Good luck and keep us updated!!