you clearly have no idea how difficult it is in the UK right now to make an appointment with a GP to ask them general questions about your personal fetish... let alone to call a proctologist! you would need to be referred to a proctologist and I imagine that's nigh on impossible unless you are in need of emergency care.
moreover, there is a fundamental problem with this entire thread, which is that a medical prolapse and a fetish prolapse are completely different things. a fetish prolapse is essentially a body modification. we're altering a specific part of our bodies in order that they appear, and function, differently to how they ordinarily would. the medical profession no doubt views the fetish prolapse much in the way they would a split tongue or stretched earlobe, or even a tattoo for that matter. show a doctor a split tongue and he'll no doubt talk you through the process of putting it back together. show them a stretched earlobe and they'll show you how to 'correct' it. show them a tattoo and they'll tell you how to remove it. it's not their job to advise you on how to get a split tongue or to stretch your earlobe, and they won't give you a tattoo either.
likewise, a doctor (if you could get in to see one) could not tell you how to 'safely' prolapse your rectum.
now, further to this, when you consider the 'risks' of a prolapsed rectum, what are they? the biggest is that it becomes trapped outside the body. strangulated, deprived of blood flow, the tissue will die (necrosis). now ask yourself: how does this happen? is it even biologically possible for this to happen to someone with a fetish prolapse? no, it is not. this is because, in literally every instance I have ever seen, read about, and experienced myself, the process of anal exploration that ends with a prolapsed rectum began with insertion. you insert something, a finger, an object of some kind, into your asshole. you enjoy the way this feels and over time this leads to the second phase which is stretching. you insert ever larger objects, more fingers, eventually a fist or even multiple fists into your asshole.
now, let's revisit the unfortunate case of the strangulated prolapse. what is it that is strangling the very life out of this poor rectum? yep, it's the anus, the anal sphincter. specifically, it's an anal sphincter that has never been stretched. so when the prolapse pops out, the anus clamps back around it with enough force to not only make it so that the prolapse cannot be pushed back in, it cuts off the blood supply in much the same way as a elastrator cuts off the blood supply to the testicles for example. now, can an anal sphincter that has been stretched to the point that it can comfortably accommodate an adult male fist clamp shut with sufficient force to cut off the blood supply to the prolapse that is protruding out of it? NO.
so, what are the risks of a fetish prolapse? whatever you do, don't bother asking a doctor.