Oh Lisa how terrible for you and what a worry.

some of what you describe sounds a bit like what my Ellie has -chronic gingivitis .

I guess it is not that , or the vet would have mentioned it.
It is treated much the same,with antibiotics, pain relief, except long term steriods and there is no cure for it,it can be caused through some of the things you wrote about this illness your cats have, and of course eating is painful and they loose weight, it is often the bacteria in their mouth that they are allergic to, that is what my vet explained to me, and there can be under lying conditions that also cause it like you listed.

I can only hope and pray that you can get the kitties well again, I have only one to deal with and that is hard enough emotionally and financially to deal with so i really have so much empathy for you.

Keeping fingers and all paws crossed for a more positive outcome. HUGS.