First of all, it helps if you've got punctuation here and there. Good god, my eyes hurt. A paragraph is a paragraph, not an entire sentence.
I hope I can understand you and help. After an eyesore of a read, it sounds like you are lacking significant internal rotation of the shoulder. The "arm-behind-the-back" is a good indicator of internal rotation of the rotator cuff. The further up you can slide your hand up, the more range of motion you have.
Use a towel with your off-hand, and grip the towel with the hand behind your back, and slowly use your off-hand to pull the arm behind your back up.
Your shoulder maybe mechanically sound, and you claim to have shoulder problems despite the lack of structural damage. It sounds like there is significant muscle imbalance. Uneven tightness within a joint can cause a lot of stress.
So, there are two things for you to do, stretch your internal rotators, followed by punctuation.