Cyn, I'm very sorry. I just read this whole thread, and I know how devastated you are.
If you don't like the scratch, get sweetfeed for horses. It's like super nutritional scratch, and they will love it. I get mine high in oats. The molasses will be good for them too. Much more healthy than chicken scratch.
If your feed store is having feed that is dusty, crumby, and buggy, the feed is old or improperly stored. It may have some bacteria or mold growing in it. Maybe you could get some feed from a bigger town where the turnover is faster at a feed store.
I would also take one or two of the chickens to a vet to do some blood tests.
IMO your chickens have some different symptoms, but they have some of the same symptoms, loss of appetite, sluggish, trouble with walking or balance, slow movement, all around the same time. My guess is that it's the same thing that's causing it.
Remember, too , that Purina had that feed recall, so feed has always been my first thought even a different brand.
I don't think I would be thinking that you would be bothering this doc you were talking to online (?), the worst thing that can happen is that no one answers you. That is not worth not trying.
I hope I've given you some more ideas, hope this gets figured out soon. Karen