That's really bad news! I've heard bad things about Banfield.
I'd contact a lawyer. Find out what can be done not only to the breeder but the Banfield as well. With their misdiagnosis, it's caused you to keep a dog that has medical problems...sorry, kids are in here and it's hard to concentrate. Anyway, like I said, I'd contact a lawyer and find out what kind of case you have against both.