anatomically you need external rotation in order for the huneral head to avoid impingement as you either abduct or flex into elevation. Without the external rotation you should never move the arm into elevation. I can't see how you can have a functional shoulder unless you maintain external rotation.
Half or full trays and positioning in abd and at least nuetral rotation in lying are helpfull in maintaining range
A sling will not prevent subluxation for you.
highmode