Tough case. It does sound neurological - almost like tardive dyskinesa but the drugs should have worked. Having ruled out most pathogens and serious neurological problems may I offer a approach of mechanical origins:
1st we know the shoulder is and has been unstable for a long time (torn ligament, AC degeneration? ect....) - This needs to be passively stabilized!
tightness will subside because the strapping will provide support meaning the muscles don't have to (as much as they would without strapping - anyway.). The one muscle that was hiking the shoulder blade (this raises the acromiom space and is common antalgic position of the shoulder) has now been made inactivated by the botox putting more stress on the romboids and will probably lead to further progression of OA in the shoulder. A bonkers thing, in my opinion, to do when there is a already established structural instability of shoulder.
2nd the romboids, levator scap all anchor the scapular to the spine. Spinal manipulation will often help these muscles to relax - if it is done at the insertion segments. I suggest you try this first as it will most likely aid recovery. there is no such thing as its just muscles or its just joints every chronic syndrome involves both the musculo and skeletal systems - have them both checked out.
3rd the torn ligament - how bad is it? will it get better from strapping? The degeneration most likely of the AC joint might be giving osteophytes in to the subacromial space creating irritation.
Question is do you need a surgical intervention in shoulder??? and is this just a heighten reaction to chronic instability? I would say it is - given that these where the only positive finding on all your tests.
yet - strangely they have not been delt with properly. If you can add the full reports of all tests that would help.
Also cant see you tube video can you put link.
Niall Marshall-Manifold
Doctor of Chiropractic