I'll answer this before reading any other replies.
I will assume that your side of the story is complete and factual - even though there are always 2 sides - but having met all three of you, IMO you are by far the most trustworthy of sources without ensuing emotions.
I commend you for keeping your cool as long as you have. Yes, the New Years was totally planned and totally bitchy by her, that is obvious. I would have have most likely confronted her on that right then.
To have a party and leave the house in pukage...that'* just wrong...however from meeting her, it'* my guess that you, my dear Geoff, do most of the cleaning anyhow.
I realize that you're on the home stretch here with college.....I would set her down, rationally and calm, and go over the rules of the house in a non-confrontational manner (in other words...Tom cannot be present ). If she is unable to clean the house back to its proper condition, then you will charge her $$ for your time to clean it - I believe housekeepers get like $60 or so to do general tidying up.
There'* no reason why you guys can't manage to stick it out for a few more months, since you've already been through for a few months now...however, all members must be held responsible for their actions and play by the rules.