Damn, sorry to see you're having issues with your shoulder.
Semi-auto shotguns are built around a delicate balance of return energy. The problem you're having with ejection is that without a solid mass at the rear of the weapon to absorb the recoil, there is nothing for the bolt to push off of hard enough to engage the extractor spring. Can you tuck the butt of the shotgun between your elbow and torso and apply pressure to keep it from moving without hurting your shoulder? If not, you might try bracing the stock against your hip...