I think you should contact your state aggie person.  Dept of Ag - Are you keeping them against any rules?  They may be more help to you and have an avian vet look them over.  I was thinking too - check for mold around the coop and/ or ammonia build up where they roost.  Too much of either can make them sick, If it is ammonia I think you can buy carbon and that will help - I don't think it will hurt the chickens - hospitals use it for poisoning sometimes.  Is it cold enough you can freeze one of them for study - I know that seems grisley but it could be important.  ammonia is really bad and in cold weather when you want to keep them warm they might not have as good of air flow and before you know it you have an ammonia gas build up from their poop.
I lost 5 parakeets one year when I was using corn cob medium in their cages and I had no idea it was molding below and they were dropping left and right!  I felt so bad.  I looked at the top but it was underneath and quite bad. I was also working a lot and gotton lax about the weekly cleaning of the cages at the time.
Do you add a little Apple cidar Vinagar to the water?
I would use a spoon and put it up to your girl's beak, for the one you have inside and see if you can get fluids down her.  Dehydration is bad too! Pouring it down her is really bad but If you get the spoon up to her beak she may drink on her own.
I will keep you in prayer and your chickee too!  I am so sorry for your losses!  I would be pulling out my hair with worry too!
Caroline