This is a list of (some of) the exercises I have started doing over the last few weeks to engage many of the shoulder-related muscles that are all "lalala, the upper trapezius can do all of my work for me forever, lalala" with some emphasis on being able to roll again*.
For all of these, your shoulder blades ought to be down and back, "in your back pocket," and the neck ought to be in neutral posture (similar to 1).
1. Head tuck/nod. Engages stabilizers that run up the neck to the skull. With your tongue on the roof of your mouth, move your head/jaw back into your neck (feels kind of opposite to the first-aid jaw lift), then gently nod down. Watch in the mirror or feel to make sure that the muscles on the front of your neck are not tensing.
1a. Stand/sit and get your head to this position. Put a resistance band behind your head; pull gently with both arms while keeping the head steady and not clenching the front neck muscles.
2. Serratus press. Lie on the floor with a free weight in your vertical arm or length of resistance band appropriately anchored. Move your shoulder and therefore the weight up using the muscles that wrap around the front of your lower ribcage (serratus anterior), and NOT your upper trapezius.
3. "Weight-shifting." Find the edge of a table or bed or something and lean on it in plank position. Push further up, engaging the serratus, to get your chest as far from the something as possible. Your upper traps are engaged somewhat but not actively working in this. Using the serratus, push up on one side to bring your weight onto one hand. If you are working on getting the "neck goes the correct way" rolling habit, turn your head to the side that is not bearing the weight. Slowly lower and repeat on the other side.
4. Similar to 3 but start from somewhere between plank and hands-and-knees on floor. Press up with one hand to take weight, lower to elbow on other side, lower to elbow on first side, one arm goes through the opposite arm-body gap with appropriate head-turn. Of course, shoulder blades are down, upper traps are as relaxed as possible, and the serratus leads most of these motions.
5. For lower traps mostly. Lie on a Swiss ball or the corner of a bed or sturdy table. Head and neck as in 1, thumbs up, palms forward. With shoulder blades down, raise hands vertically (following thumbs), squeeze shoulder blades together, do not shrug shoulders. Lower angles between arm and trunk are easier; 90 deg or more is much harder.
6. Plank from knees, resistance band around hands, hold the serratus press position from 4. Push outward against the band. This works some rotator cuff stuff too.
7. General stabilizer work. Resistance band around hands, hold them in front of you at various angles, push out against the band. Also, hold overhead and push out, and hands behind back and push out.
These have been really useful so far but obnoxious, especially some of the ones my new PT is giving me. Still, not much point in getting good advice if I'm not going to follow it, and I can tell that being able to actively engage mid-trunk muscles is going to do excellent things for connecting my hips to my shoulders. PT says it is a good idea to keep up with the stabilizer work even after I am healed, since my shoulders are so very flexible.
*I tried rolling on the squishy mat today and found that it is easy to develop bad habits because of the give. For instance, if you can't really feel when you go over your head, that's a terrible one to take out on a firmer surface. However, I have started rolling from standing on the normal mat and it's been a bit bumpy sometimes but not terrible.