Have you tried Preparation H to shrink the tissues? I had an old hen with a prolapse for a week or so. Her shells became bad when she got up over 10 years old. Urates would dry on the protrusion and we had to put her in an epsom salts bath to loosen it up and it literally came off in a shell. Then, we kept gently pushing it back into the cloaca, soaking again, pushing, then more PrepH, kept her in a semi dark area, etc. It's just one of those things that takes persistence and gentleness. My hen's shells crushed inside and the shell would push out with her straining and we had to gently tug the rest of it out. It takes a lot of time, but is not always successful.