Jerram,
I must admit I'm still a bit confused. Since the proximal arm was scanned, no conclusion can be made radiographically about the distal part. Secondly, having pain at max ER at about the area of subscapularis insertion could also be that other structures might be involved, like glenohumeral ligaments or the capsule (maximally stretched at maximum ER). Since at onset you had pain with resisted ER and subscapularis is an internal rotator, it couldn't be involved. Perform manual muscle test for subscapularis (arm at the back and resisted IR) to rule this out. Infraspinatus and teres minor, both external rotators, could be involved, but again these attach to the greater tuberosity, not the lesser. And they are not the only external rotators. Therefore, shoulder rotation is kind of confusing concerning symptoms and area of pain.
A comment about biceps brachialis... biceps is a strong forearm supinator, stronger than elbow flexor. So, its muscle test should be focused on supination first (from full pronation to full supination with elbow flexed at 70-90 degrees) and elbow flexion then. At the shoulder joint, it flexes and slightly internally rotates it. So, if during shoulder flexion, ER and forearm supination you get pain at the anterior shoulder, then it could be biceps. With IR and pronation, you should feel less pain since it's anterior band of deltoid that works harder. Check again these movements (supination with elbow flexion-classical manual muscle test for biceps-, shoulder flexion with ER shoulder and supinated forearm-for biceps- and with IR shoulder and pronated forearm-for anterior deltoid) and inform us of the symptoms and area of pain.
ilias