I don't know if you're interested in reading an entire book, but there's one called "The Dog Listener" by Jan Ferrell. If you read her book, it sounds like some of the things she does are nothing short of miraculous, but I do think her primary premise is excellent. I've tried using her techniques with my dog and they work. She's got a chapter on a dog that chews on its paws.

I sometimes think dogs who self multilate are bored too, as has been suggested. Could you let her stay in the house outside of her crate? I would think she's probably reasonably trained by now. Some dogs like to watch Animal Planet. A frozen kong with peanut butter inside it might be more attractive than her paws.

Also, most vets have lists of dogs that have been found and some have already been spayed. I would think you could find another one that doesn't need to be spayed or neutered which might cut the cost considerably.