I know Vet bills are not cheap, but wow, this does not seem to have an end.
One fecal sample on the typical bird that died would probably give you the answer.
With all the time and effort you have given your birds it may be worth the cost to find out just what is doing this to them.
Then...
Is there anyway to move the remaining chickens out of where you have them. To a temporary shelter?
I am wondering if something is biting them. Could you have poisonous insects? Something creeping in there at night and feasting?
I know I see mice in my coop at times. And black widows. Living in...
Have you checked the local feed stores? Mine has corid (it is also used for cattle and that is the section they had it in)
I could only get the 16 oz. bottle. You can order online too.
I would treat them with that and or Sulmet. I don't see how it is going to make matters worse.
I am so...