Ouch! I feel for you (literally, when I hear about shoulder injuries I think mine gets some sympathy pain ) I broke my humerus several years ago, and while I did not need surgery it was touch and go for awhile (they xrayed me twice a week for over a month). I have to say, they originally thought I did what you did (sounds like you basically shattered the head?) so I feel lucky. Mine was just below the head, but a complete fracture with massive displacement. I lucked out that with some time the pieces lined back enough to avoid surgery.
I didn't start rehab for almost 6 weeks b/c they waited for the bones to stabilize, so my shoulder had really frozen up. It was a very long process, but I got back to riding and have had no ill effects long term (the elbow I recently obliterated is another story right now ). I don't remember having any issues with jumping or xc once I got back to that. I think I was back to lunging in 12 weeks, and then riding with reins a few weeks after that (that entire side of my body was quite weak at first).
I was told that if I did have surgery, I would have been able to start therapy much sooner so my return to riding might have been quicker. One thing I recommend that neither my PT nor my ortho focused much on is make sure you rebuild the entire side and not just the arm/shoulder- I did notice when I ramped up my riding that my left side (I broke my left humerus), lats, abs, everything on that side of my core were much weaker.
Good luck with your recovery! If you need to chat or vent, let me know. Rehabbing a shoulder is tough, but worth it!