Would be interested in an update from this case study.
I have been trying to troubleshoot similar symptoms for months if not going on a couple years now.
Mostly, I present posterior shoulder pain when hanging from a pull-up bar (more pronounced with a supinated grip) and can “pop” my shoulder by internally rotating my humerus or, interestingly, retracting my scapula and flexing the humerus. The “popping” becomes more pronounced with increased abduction (former case) or retraction (latter case).
Interestingly, any kind of rowing movement is most problematic as it increases my “need” to pop my shoulder (which I see as detrimental but necessary in that it is extremely uncomfortable to walk around in a state of hypertonicity).
Also, I have noticed working on the soft-tissue of downward rotators of the scapula (rhomboids, lats, levator scapula) with various smashing and band-mob techniques and then doing some light overhead dumbbell pressing helps my symptoms but the relief is relatively short-lived (~24 hours).
I would love to get back to being able to train my upper back and throw a baseball again pain-free.