Re: Horizontal External Rotation Pain
Does sound like maybe some supraspinatus irritation there + infra especially with that indication of intense pain on the cubans.
Lots of massage, tennis ball it.
I'd recommend trying to get into your shoulder with 30+ mins of massage per day. More if you can. Try to hit all of the muscles around the scapula, neck and delts as well including the pecs.
If there's swelling the supraspinatus ice would be good, but can't really tell.
How's the posture looking?