I tend to agree that the mechanics of the shoulder joint are important. That said a true tendinitis is possible and could be diagnosed via a combination of Wikipedia reference-linkMRI and a reduction of pain with an injection of local anesthetic into the tendon area. I find most tendinitis resolves with time and you are better off focusing on cervical mobs and neural mobilisations etc.

Releasing the subscapularis under the axilla is often very useful in reducing pain and increasing range of motion. Best of luck 8o