@microchick
I understand your pain and frustration and I hope my response didn't come across as unsympathetic or critical.... feeding treats can very easily get out of hand without people realising there is a problem.... I have to check myself regularly too on this. After all, feeding dry bread doesn't even seem like a treat to us.
I would be concerned about mixing the scratch in with their regular feed as they are notorious for picking out the bits they like and sometimes they will bill out and waste the other stuff to get to it or the more dominant hens, possibly the "fat and sassy" ones (sorry but I absolutely love that phrase and just had to use it again) will have first dibs and pick out the scratch, leaving the other stuff for the lower pecking order birds.
The problem can be that people's perception of "fat" varies and many people don't recognise it in themselves or their children or their pets and some cultures actually see being "fat" as a good thing.... probably because in those cultures, only rich people have access to enough food to be fat...... anyway, I digress..... The problem with chickens is that it is very difficult, with all those feathers, to assess whether they are carrying too much
fat or not enough
flesh, but clearly you are monitoring that situation due to the Marek's. The other thing to be aware of, is that production birds will never be anywhere near as well covered as meat birds and the condition score chart that is occasionally used here on BYC is, I believe, angled more towards meat birds. If you have ever processed laying birds, even DP birds, you come to realise that they are naturally pretty scrawny compared to the plumpness of meat birds. Most of my layer breeds, hens and cock birds score a 2 on that chart. It is not anything to worry about and I don't try to fatten them up. Anyway, what I am saying is to be sure that their body condition is actually a problem, as your expectation may be too high for them.
It is a shame that you don't feel able to open them up, as that really is the only way to know for sure what is going on in cases like this. Without knowing what is going on inside all we can do is speculate about possible causes. In some instances it has enabled me to change the management of the rest of my flock for their benefit from what I have found, but I entirely understand that it is not something everyone can get their head around.
I have not personally come across anything linking prolapse to Marek's, but it is a disease with so many different facets that many symptoms are overlooked and like you, most people do not get/do necropsies, so that information may be lost.... prolapse is an obvious cause of death in itself, so the majority of people would not think to look for an underlying cause. I do think you are right though as regards it not being a tumour. Usually, once they get to that stage, they have started wasting away and from what you say, that was not the case with her. She may have had a heart attack as a result of straining with the prolapse, particularly if she had fatty deposits around her organs as I have seen on birds that got too much scratch. Again I am speculating.
If it was me, I would go back to a grower or flock raiser pellet as I think your game bird mix may be creating more problems than it is solving and scatter a small amount of scratch for them to forage in the run (especially if you have deep litter)..... which is it's intended use..... thereby giving them something to occupy themselves with and exercise in "scratching" to obtain the treat, negating the fat build up from it.