Okay since no one will read a novel, nor do I desire to post one, my post will raise mote questions than it answers. That's fine, I welcome Any specific questions.
#1 muscular balance. I do about 3-400 reps of upper back and rear delts per week. Apiece. Not lumped together.
#2 There are 17 muscles which attach to the scapula. Most of them are small and not very strong, but all of their jobs are important. So when you're only using 4,6,8 of them, your shoulder mobility is ass. Just activating more of them will improve shoulder mobility. In a sense, shoulder mobility and health is a learned skill. Getting better rhythm and activation out of those muscles will improve shoulder health and mobility.
2A.) Do not try to pinpoint the exact muscle holding you back. You don't have a Ph.D on the shoulder nor do you have eyes in the back of your head. Plus, like I said, 17 muscles in a small area, all running in slightly different directions. What chance do you have of pinpointing? How much time do you have to guess and check one muscle at a time? Not enough. There are moves that address the complex of muscles and the rhythm that will sort you out pretty well. Mace swings, KB around the worlds, and the band warm up I will edit in the bottom of the post are some examples.
#3 Thoracic extension. If you have it, your shoulders are happier than if you don't.
#4 Tight biceps can be a major cause (see where the origin of the bicep is located and this will make more sense).
5.) learn how to pack your shoulder better and use it for everything you do and things will work better.
6. Learn how to breathe properly and your shoulders will open up. You can literally gain five degrees internal and external rotation during a proper breath. See "Breathing 101" by Juggernaut on YouTube to learn more about this.
7. Learn how to do Donnie Thompson's "bowtie" and do it often.
Bonus: Do this before every training session and in a few weeks you'll be FAR better than when you started. Then keep doing it before every training session until you die.
https://youtu.be/txrvbXmqbcY