If you want an inherently safe way to rehab your shoulders, isometric contraction while stretching/reaching/twisting is protected from serious injury by natural reflexes.
A bonus of these natural reflexes (proprioception) is that it also helps a lot to stretch and improve range of motion. When the muscle movers one side of a joint contract, their partners on the other side naturally (reflexively) relax and extend.
Tightness on one side of a joint, is almost always paired with weakness on the other side - two sides of the same problem. Both need to be developed, to regain full, pain-free range of motion.
The whole area around the shoulder joint - the shoulder blade, the neck, rib cage and thoracic (chest area) spine tug on each other and contort, when one part gets tight and shortened, or strained and weakened.
So pull all the slack out of the whole system (from head to foot, and from hand to hand) by extending the spine tall (as if being lifted from the top of your head, like a Christmas tree ornament), reach with both hands far out to the opposite sides, externally rotate your arms fully (palms up), and lay your shoulder blades flat (no “chicken wings” poking out), and pull those shoulder blades down strongly away from yours ears (don’t let shoulders hunch up).
Then rotate your upper torso 90 degrees without turning your hips or head, so your arms would be stretched front to back, rather than side to side. In that position, stretch/pull/reach/twist to improve reach of each element of the position at the same time time. Then twist the torso the other way, so the opposite hand is out front, and tense/stretch the other side.
By raising your chest/sternum (like Superman), fully extending your thoratic spine (no slumping widow’s hump) your head can reach backward like a Ballroom Dancer, without bending your neck - the whole straight neck is tilted from the raised chest (careful not to bend your lower back while trying to get the head to lean back). Trying to lift your whole ribcage an inch, helps keep the lower back from arcing keep the lower back straight and stretched tall.
This stretch is actually from Ballroom Dancing, but the anatomy of it is common to all - full extension and twisting of the spine, full distraction (reaching out from the joints, like traction) and external rotation (twisting) of the shoulder joint, and fully depressed shoulder blades.
You won’t rip anything loose while working to perfect that posture, and when you do perfect it, your shoulder will essentially be fixed (as well as neck and shoulder blades - even breathing comes easier, and the heart is a little freer to beat).
...externally rotate your arms fully (palms up)...
I am not able to rotate the hand of my injured shoulder more than 3/4 up.
And then this next part I don't understand where you're saying to stretch:
Then rotate your upper torso 90 degrees without turning your hips or head, so your arms would be stretched front to back, rather than side to side. In that position, stretch/pull/reach/twist to improve reach of each element of the position at the same time time...