“How long does a dislocated/subluxated shoulder continue to improve in terms of stability and range of motion”
- That depends on the amount of damage sustained in the dislocation - ie was the labrum, or capsule and/or supporting ligaments torn away or just stretched. There are degrees of instability and different directions of instability. It also depends on your activity level and whether or not you limit the offending motions during healing, or whether you continue to stress the weakened tissues.
“if strengthened via normal weightlifting”
- Normal weight lifting does not necessarily rehab an unstable shoulder. While it may help to strengthen the shoulder muscles in general, normal weight lifting does not address neuro-muscular control and dynamic stabilization of the shoulder complex. Also, some weight lifting exercises can actually aggravate the instability. Overhead or military presses, bench presses, or wide grip lat pull downs esp. with the bar behind the head, and the squat with the bar behind the shoulder girdle for example, need to be modified to avoid over stressing the joint.
“stretched with normal stretching exercises”
- Some of the normal shoulder stretching exercises may add to your instability. With anterior instability for example, pec stretches can strain the front of the joint.
"and not reinjured?"
- Each recurrent subluxation can stretch the supporting structures more or increase a tear.
“If it's unstable now, is there still a decent chance it will gradually improve even after 9 months?”
- Again this depends on the degree and nature of the instability. But to get the improvement you want, imo you will need professional help so that you have a good exercise program to follow and avoid impeding your healing process. Hoping for the best works better with specificity of exercise.
Since you are considering surgery, your doctor will be able to answer your questions as to the degree of instability and whether or not you have a tear. If you have not already had physical therapy it is worth having an evaluation and rehab, or at least getting instruction on specific exercises for your condition. Try to go to a therapist who is either an OCS or SCS (orthopedic or sports certified specialist). Your doctor or prospective surgeon would give you the best recommendation.
Your rehab will address exercises to retrain the shoulder, rotator cuff and shoulder blade stabilizing muscles, and improve the neuro-muscular control, not just static strength. When you are ready, your therapist can evaluate your throwing technique, make corrections if appropriate and give you a progression to follow. Good luck.