I know absolutely nothing about prolapses, I hope that you can find an answer soon.
I do know some about straining though. My girl had terrible straining problems. She has weak abdominal muscle, and it also turned out that she had a colstridium infection, which made her strain. She was treated with amoxicillin and metro, but I also had to modify her diet and remove all grains, and slowly add apple pectin (soluble fiber), and slowly added probiotics. I would first look and see if you can make some dietary changes (some dogs do well with pumpkin). I would hold off on the probiotics for now, since it might firm things up and make eliminating even harder. I think I would start by feeding something that is grain free, and slowly add some soluble fiber (I like apple pectin). I would maybe make sure that there are good fats in the diet too, adding a touch of fish oil maybe, probably better yet, some virgin, unprocessed coconut oil. Again, I don't know anything about this condition, but just thinking about maybe trying to make the eliminating easier.