um, i'm not about to read 5 pages to see if someone's already said this, but:
hey, OP, listen up
PT is out there to help you
if you broke your leg "because of PT" it almost certainly was not "because of PT", if your therapy was apparently what caused your leg to break by walking, why was it in a "million pieces" and why would not walking have helped said "million pieces" to blow apart
i realize that you should probably be listened to when you scream "stop" but seriously the outbreak was pretty unnecessary if it's true.. you should tell her that you should probably develop a bit slower than she's going with the stretches because if you're feeling like your arm will split you are definitely not "fine" and do in fact need to "get better". she is in fact there to help, and while it might not seem like it, it's true.
i realize you're terrified of your mom but talk to her and tell her to tell the PT that the stretching that she's performing is too physically straining for your arm and express that your arm "feels like it will break" and that you should take it slower.. while it might mean more weeks with the PT it certainly will feel better.
also i realize that you have osteoporosis but seriously that's all the more reason to let her help