p.s. Hope that the teacher doesn’t mind my attempt to verify a few points as other quoted his drill.
__________________
If you cannot take the shoulder down the clubshaft plane, you must take along some other path and add compensations - now, instead of one motion to remember, you wind up with at least two!
My theory is that BM gets his head too far to the right during the mid-backswing because he starts off with his head too far over to the right at address. I suspect that he keeps his pelvis central and then tilts his upper spine to the right at address. That moves his head to the right of center of his stance. Then, when he rotates his pelvis in the mid-backswing, his head moves further rightwards.
I think that he would be better off if he followed Yoda's advice for getting rightwards spinal tilt at address - keep the head centralised between the feet and then shift the pelvis sightly to the left.