Good to hear yours are doing better. I'm still losing birds, I've just given up on mine. I got mine from Central Hatchery, so they are not the same batch of birds. Between all the people who bought birds from the order I made, there are 14 birds left alive out of the original 208! Since I bought them and then sold them to other people, I worry now about my reputation as a breeder, hopefully I made it clear enough to the people that I did not breed these birds, I bought them for resale. No one has demanded a refund from me, thankfully, and between several households that bought them, all the birds are having the same problem, even though they are on different management systems.
At 7 weeks old, mine are still under a pound in weight though they are eating an incredible amount of feed. No sign of parasites, predators, or infection, but they're like you said, literally just falling over dead. I notice they seem to get sluggish, and at that point their little feet and faces are cold to the touch even in an 80+ degree barn, and their feet are swollen and bruised looking; to me it seems like a problem with circulation, like the heart is not pumping right and the blood is backing up. I had a horse die of heart failure some time back, and she had similar symptoms, cold to the touch and swollen legs. All I can think of it they've got a congenital heart defect maybe, because they are not heavy enough to be dying from the typical heart failure. I have bantam breed chicks the same age that are bigger than these birds! I'm going to contact the hatchery tomorrow, and send them some pictures of mine and see if they have any ideas.