Tell her the deposit is nonrefundable. Even if they "can afford to take the hit" doesn't mean that she'll enjoy it. That might at least buy you some time.
Or just tell her that the dorm problem could be solved by having your own place in a college apartment. Or that you've decided to stay with me. Or that you've decided to study abroad next semester and that would put you at least one ocean away.
And she says *I'm* insane. Well, not so much says but, you know. I'm sorry. Hopefully, she'll tone it down once you're actually there. When's the move in date? Have you gotten your roommate assignment yet?