Former hospitalist, now retired.
I would recommend bodyweight and dumbbell exercises that address single limb in order to strengthen any weaknesses . The goal is to stabilize and equalize strength on both sides of the body. If I was you I would graduate myself from beginner trails to enduro to downhill over the course of a month or so. Jumping right into downhill is possible but it doesn’t feel right because of the surgery and downhill is downhill. One jammed wheel and your not strong enough to keep it straight…
Its not just the 1st rib or the shoulder that I am concerned about. It is the rotator cuff. With the bw and dumbbell exercises I would particularly focus on the back dividing them simply into upper and lower back exercises. I think by graduating yourself with the riding and strengthening the affected muscles and tendons as well as supports, you will be ahead.
Diet wise please make sure you are taking a Multivitamin and for now you should up the protein. Your primary physician can give you more insight. The evisceration feeling is your body telling you you’re doing it too fast.