I'd like to hug both you and your vet for being willing to go the extra mile for your sweet 'Biscuit'. So many vets around us will not touch poultry. Our nearest is in Columbia at the school of Veterinary Medicine which is quite a drive for us if we are hauling a sick bird.
After reading this thread fully I have to ask if you had your birds vaccinated for Marek's Disease? You said you lost a bird to a short illness before Biscuit fell ill from what now suspiciously sounds like a reproductive infection or cancer both of which are seen in Marek's disease. I hope it isn't that. But it might be something you may want to ask your vet to be on the lookout for.
I'm so sorry for your loss. Your girls are so lucky to have you as their chicken mom and no, do not apologize for shedding tears over your flock. I've cried a river over mine during the almost 5 years that I've been keeping chickens..