I feel it’s a bracing issue of the torso and shoulders.
Thanks for the insight into what your DD went through. It does sound like our kids had a similar diagnosis. Our Dr. recommended 6 weeks of rest which she did, but she still reports intermittent mild pain, mostly throwing overhand rather than pitching. It would seem we didn't take enough time off. I do think overuse is a major factor in my DD's issue. She pitched 120 inns. in HS last Fall, then instead of resting, we felt compelled to do some camps over the winter (that was likely a mistake) and her mechanical issue cropped up by Feb. The pain presented much later, but as you mentioned the weakness could've led to other parts of the arm (ulna) picking up the slack and here we are. Once the mechanical issue occurred, she was definitely not overused, I think she had no more than 30 inn. from Feb-July, but the pain started anyway. She is not pitching much in HS now and it sounds like prolonged rest is the most immediate course of action. The timing is brutal since the schools we're currently talking to will move on, but it is what it is I suppose.