I think venting is good. Sometimes it helps in finding asolution.
You didn't ask for advise or suggestions, and you're already kicking yourself too much. Even said, this is one time you could say, on second thought, that with your fibromyalgia and carple tunnel syndrome you do not have the energy to deal with this untrained JR. It doesn't matter how cute she is. You might add that they can take her to a pet hotel (where she just might get the training she needs) or keep her home and hire a pet sitter. If she messes on their things that's their problem. It really isn't your responsibility to train their dog.
You are scheduled for surgery, are't you? Are they going to help you then?
And yes, I've had my issues of not being able to say no, and I have family members who show up only when they want something! :angry:
Good luck with whatever you do.