Hi johnmerrick
It is a bit hard to give advice for something like this over the internet. You have made a good attempt at describing it but there is nothing like actually seeing the posture of the shoulder and see what happens when you move in certain directions.
However from what you described - slightly raised, abducted scapular - with a bit of winging. The key muscle to strengthen and get to work in a more shortened position is the lower trapezius muscle. With a long standing problem like this I would expect strengthening the weakened muscle(s) is going to be more effective than trying to stretch out tightened muscles. Building up the strength of the lower trapezius should make it easier to conteract and provide passive resistance to the tightened muscles at the front of the shoulder.
It is rare to get spasm in the serratus anterior. Usually the reason that the scapula is out to the side, around the rib cage and there is some some winging is a tight and/or over developed pectoralis major. If there is a bit of winging of the lower part of the shoulder blade of the pectoralis minor is often tight. Tightness or spasm of the serratus anterior would cause the scapula to abduct but there would be no winging
Does your therapist use a movement impairment syndrome approach (author is Shirley Sahrmann)? That approach often can offer help with this sort of thing.
One other possible reason is if there is some adverse tension of the brachial plexus (nerve complex that runs between the cervical spine and the arm). A key symptom might be pins and needles or numbness down the arm. However this really has to be checked out by a skilled therapist.