I think you're right to consider culling her, Roisin. Given her poor laying history, and the prolapse, it's highly likely that she has some internal damage or problem which is going to get worse whatever you do, and it's unlikely she will ever lay satisfactorily. Yes you could spend a lot of money at the vet, if you could find a suitable and experienced one to operate on her, and it might work for a while, but I don't personally think it would be kind to put her through that, plus the isolation etc she would need whilst recovering from the shock. Home treatments are also invasive and probably not a long term cure. If she did come back into lay it would be highly likely to recur, she might be attacked by the others before you noticed what had happened, and in any case would have to put up with a painful and debilitating condition. I know many people go to great lengths to keep their pet hens alive come what may, but one always has to consider whether this is either kind or sensible as a long term treatment,
I give my hens a really good life, and when they succumb to a potentially terminal condition, they are culled, simple as that. Trouble is, they all seem to live to a great age as healthy pensioners ....