I have a hen that is very near to passing..I read the whole thing, but I still can't figure out how to get her necropsy..
I would not wash the carcass personally... and that was mot suggested to me by the state vet at UC Davis when I was in contact with them.
How to Send a Bird for a Necropsy
State poultry labs

Dear OP, sorry you face this.

My suspicion would be Marek's first coccidiosis second. Biggest suggestion... get answers and get a necropsy done by the state vet. Many local vets will do it, but they are much less detailed.. and I have SEEN them accept the first answer and not a true work up for the whole answer.

Hang in there!

ETA: worming an already ill bird if that isn't their problem isn't going to help them. Get a fecal float first and treat what you have instead of what you don't.