Perhaps you could consider the axillary nerve as a potential area to investigate. The dermatome of this is close the the insertions of the rotator cuff into the lateral humerus. A traction injury could account for a lesion in the area. You transient relief from the subscap. release is a good indicator to continue on that track.
Perhaps additionally look for active "trigger points" or more casually painful spots in the infraspinatus and teres muscles and you can use acupressure to release these.
I would also begin some eccentric internal rotation. You can achieve this with the client in sitting and their elbow resting on a table at about 70 degrees to the side of the body (slightly forward of the coronal plane "Scaption" not abduction). Then bend the elbow to 90 degrees and have the patient perform active external rotation exercises of the humerus with focus on the eccentric contraction (of the external rotators) as the arm moves back to end of range internal rotation. You can progress reps, then speed, then without elbow support before you increase the weight. You can also do this with theraband resistance. holding one end and standing on the other. To assist depression of the humeral head while performing the exercise they can gently press the elbow into the supporting surface.
One thing, don't worry about slight neural discomfort in the area of the rotator cuff attachments when initially performing this exercise. This is common and with resolve after a few days as their control increases. Best of luck 8o