Hi
Not sure if you have received reply to your regarding scar tissue in your shoulder. If so, I would certainly like to know the outcome. I 'believe' I have exactly the same problem.
I had a shoulder surgery in June 2008 for what was supposed to impingment. According to Doctor, surgery was perfect and my shoulder should have been ok within no more than 2 months. But now after almost 14 months, I still have pain in movement and I can feel as if something or some muscle inside is very tight (even without any movement), the same feeling one gets when you have a cut on your outer skin and dry skin covers the wound. I believe that is what Scar Tissue is. As in your case, MRIs show no signs of any damage to shoulder, muscles, ligaments, tendons etc. It is becoming quite frustrating.
Manish