On one hand, I know you are disappointed at what happened , but if you love your dog the way most of us do, I know you're glad to have her back, regardless. I know that things like this happen, hopefully rarely, but was it incompetence or just a chip that was hard to scan? Did your dog have on a tag? It is amazing that they operated and placed the dog for adoption in such a short amount of time. Maybe you could focus your energies on revising their system, rather than hiring a lawyer. You're not going to get back what you have lost that way.
I have two purebred Golden Retrievers that I chose to have spayed as early as possible. I simply wasn't up to the "puppy" task.
Sorry for your trouble. I was just trying to think of a way to turn this into something positive. It could be your MISSION!!