Good luck. The lead orthopedic here prescribes pillows like that to many of the patients that are having labrum/rotator surgery, shoulder fractures and excessive arthritis. The majority dislike the pillow because of the elevation it has once your arm is tunneled through. There is a company here that buys them back and other things like wheelchairs and knee scooters.
I am indeed someone who has had incurred a shoulder problem this year and has had major issues with sleeping and that is absolutely what happened to me using a tunnel system. You can get very shallow ones that don't add massive elevation but it didn't really work for me and I tried a few wedges and all sorts but that was a big fat fail on that.
I envy anybody who can change the way they sleep - it has been incredibly hard to work through. I sleep on my stomach predominantly and the shoulder is probably more pivotal than any other sleeping position. Of course, I tried to get away with switching to falling asleep back on the back but try as I might, that isn't happening. Though I will wake up quite often on my back. Just will not work for falling asleep.
I've basically muddled through with a combination of sleeping a lot more on the other side, occasional side sleeping, occasional back sleeping (possible later on in the sleep for the final portion) and quite strangely, the other thing I've found is that basically doing the arm out but elevating the hand on a pillow actually might be better than the tunnel. I didn't read that anywhere, I just sort of found out that elevated arm works better for me. I do that with my better shoulder all the time.