Insights on Frozen Shoulder
Having had two experiences with frozen shoulder, one on each side, it's been quite a learning process in how to resolve them.
The left side is still clearing out and was the result of a head injury that effected everything on that side. Since the neck took much of the brunt of the impact on the head, the nerves in the neck over-stimulated and locked the shoulder, the muscles in the arm, down the back, and the areas that I didn't count on - in between the ribs and chest. It was a pretty complex process and it won't work the same for everyone, but here goes:
On both shoulders, the 'pinching' seems to arise from the middle deltoid, which makes me wonder what there is underneath the joint that might be restricting it from reaching or lifting. The latissimus dorsi is the most obvious choice since it wraps around from behind and attaches right at the top of the humerus along with the teres, but that didn't help. Loosening the rotator cuff helped a lot, returning 25% of the range of motion, as did working with the subclavian muscle (under the clavicle), the sterno-clavicular joint and acromioclavicular joints. Those get compressed just sleeping on your side. Releasing the brachialis muscle that connects to the deltoid was also very helpful.
It might be the result of carrying so many purses and gym bags on my shoulder for so long, but the compression between the upper ribs was a huge factor. There was also a time when I did regular push-ups and the serratus muscles were in near spasm but without sensation until you touch them. There are quite a few lymph nodes in that area also that were compressed. Taking the pressure off those uppermost ribs and clearing the muscles in between them was the most helpful, including the pec minors, allowing the return of the last 5-10% of the range of motion. In this case, the left shoulder joint was trapped/frozen by muscle tension and spasm stimulated by the head trauma. In the right shoulder, the capsule itself was adhered from a few small insults to the joint, but the symptoms were very similar and the path to restoration was less complex, but also similar.