There are people that know about breeds that are resistant, but I'm not one of them, lol, though I have read that Silkies and Sebrights are *very* susceptible to it.
I try not to hatch chicks because almost all are just mutts, and I don't need more mutts. Last October I won a BYC contest and my prize was 10 chicks from my pet chicken. On November 8 I received 11 un-vaccinated rainbow chicks (or whatever they are called). Now they almost 7 months old and so...
I've tried numerous times to save them, and some will get better for a little while, but they always relapse. In hindsight I should have euthanized them, and that's what I do now when their quality of life deteriorates. Hugs to you, because I know how traumatic this can be.
More on my reasoning... The one doing the splits will almost certainly have tumors on it's sciatic nerve, so there would be no good reason to pay for the histopathology since the "gross necropsy" will be all you need. The one with the green poop could have something else going on that a gross...