I just got to thinking about this some more and thought of something important.
Sherry, if you decide to pursue this legally - and I definitely think you should, you are undoubtedly going to be portrayed as the bad guy for asking to have Petra euthanized. Please, remember first of all that you have all of us behind you to not only support you, but testify to the fact that you tried everything you could and explored every possible option for Petra.
Go back and print out every single post on every single thread that mentions your problems, successes and setbacks with Petra to back up that this was not an easy decision for you.
Unfortunately, this is not going to get any easier for you, but Petra - and the overnight attendant who called you - are heroes for bringing the unethical practices of this vet clinic into the light. I sure hope that woman doesn't get fired for blowing the whistle. Petra was sent to you for a reason. This must be it. Embrace it and let us help you any way we can.