First of all, I am sorry for your multiple multiple losses, and hopefully you will have an answer soon you can live with.
Second, I feel the strong need to let you know- I think part or all of the problem lies in your open farm situation- if this turns out to be an infectious disease. In your list of how many chickens you have lost in the last year, you mention multiple visits to auctions & swaps. You brought multiple birds home from places which are a mixing ground for disease. Swaps & auctions are FUN, but actually a really great place to pick up diseases- like carrier birds and incubating birds. You also continued to sell birds during several months you were incurring deaths from this unknown cause. You were unknowingly or unwittingly are potentially spreading whatever this disease is to other peoples flocks. Maybe this is just a toxin or poison on your property- but if this is one of the nasty respiratory bugs- you have likely sold carriers or incubating birds to other people- and the cycle will continue. I know you would not willingly have someone else go through what you are going through. If you have not already done so, I really really recommend closing your flock-- NONE in NONE out, until you know what you are dealing with and have a solid plan in place. In the future, I would also seriously reconsider your buying habits at auction. Safest thing to bring in is chicks or hatching eggs from good hatcheries or breeders. Most dangerous is to bring in started or adult birds in from swaps IMO. Again, hope things turn around soon & you get some good news.