Again, I agree with "Stina" - a breed-specific rescue group would be absolutely certain that the dog would go to the appropriate home. We have a Greyhound rescue home here that has the most very stringent qualifications for prospective adopters...very difficult to adopt these dogs!
This dog seems like more of an untrained animal situation, and, to me, it's not fair to "take it out" because it's not possible, in this situaton, to train it properly. Let someone who has the time and the will and the know-how give it a chance. If a professional believes it's an incurable biter, then so be it. Seems to me like an animal that has not adjusted to it's new life situation. Big changes aren't easy for anyone. Really - I'm not trying to sound anthropomorphic - works the same way with a new chicken in the coop, right? This dog needs training in a situation where it doesn't feel threatened, yet learns it's place in the family.
I truly hope you're able to let it go to someone that may be able to help it. 6 years is too young to give up on.