Update:
So far so good. Supple Leopard has a very good list of exercises for impingement. It has been very helpful. Using a set of back exercises and chest stretches, I have significantly reduced the pain, and increased mobility and strength.
There is a specific exercise that I do several times per day. With straight arms I bend over and push down on the back of a chair. Keeping strong pressure on the back of the chair, I walk backwards until I’m fully bend over with my arms straight out in front. Then I walk back forward, still pushing down, until I’m upright. This provides instant, fairly long lasting relief, and or strangely keeps my shoulders pulled back after the exercise is over. Photo included for clarification.
I do a lot of chest stretches, and spend a lot of time laying on my back on a roller with my arms draped back.
I am also constantly drawing my shoulders back and fixing my posture, throughout the day and during every exercise. Icing in the evening and taking anti-inflammatories also seems to help.
After weeks of doing PT and getting no guidance on what to do on my own to help, I am much happier with the progress I’m getting on my own.
This is unique to me, and I am lucky that I didn’t have further damage. I have experience rehab’ing back and knee injuries, so I was confident that I wasn’t causing further damage. I my PT also reassured that I wasn’t at serious risk for further injury, so YMMV
Going forward, I have some plans to prevent this from happening again. More back exercises and better posture, obviously. I also recognized that this coincided with a huge advancement in my squat, and I am thinking that torquing into position for squats was a contributing factor. A Safety Squat Bar or cambered bar may be in my future. I think that a cambered bar may put me in a much more comfortable position for a wide range of exercises.