What can I do about my nightmare housemate?

As hard as it may be, I would keep all my things in my room, including my T-paper. I would also collect her part of the bills before she leaves or make her sign a promissory note. If the other room mates decide to let her use their things, then they can clean up after her. You don't have to, because your things are not being used. Good luck to you. As for the mess she leaves. Leave it at her door, she'll HAVE to pick it up.