So, updates???

I posted your dilemna on another board, and these are the replies I got:

If she's able to eat/drink, it probably isn't a foreign body. Sounds suspiciously like a mass-abscess or tumor. If it were an abscess on the outside of the jaw, I would think the biopsy would have released pus. Abscess could be on the inside of the jaw. I don't know how much help an xray would be, as a rats head is pretty small-might not reveal enough detail--but they could try that too.
my cat had that happen once. It was follicular infection. It was very large swelling that looked like a tumor. I don't know if this happens in rats. We used peroxide and neosporin to fix it up. Has to squeeze hard to get all the puss and infection out. This may or may not help.