I wonder if your first vet suspected Mareks disease? Did he ever say that? That could be why he was so pessimistic and wanted to euthanize her. Is she standing or walking at all? Pleas let us know what this vet says.
I would concentrate on getting the Corid water in them mostly. If it is cocci, the gut is very painful and not working well--that is why they stop eating. When the Corid starts working, hopefully you should see some improvement. Can you call your vet and ask the clerk if there was a...
How is the ventilation in your coop? Do they go outside in the daytime to range in the yard? Runny yellow poop can be a sign of cocci, but if it is bright yellow it could also be a sign of E.coli infection. I would treat with Corid, and if Corid or Ampromed is not available, use...
Do she have any rattles when she breathes, coughing, sneezing, watery eyes or bubbles, nasal drainage, or swelling around an eye or the face? Does she gape or open her mouth to breathe? What did the vet say it was? Coccidiosis is very common in flock of various ages, and comes from the soil...