There's always a chance chooks won't eat a food they don't recognize even if it's good for them. After the Fukushima issue my chooks went off Chinese garlic, which was what was readily available in shops, and they've never gone back, despite me trying them multiple on the premise that it's unlikely it's THAT irradiated. Many things can be done to any natural food source that can rob it of its inherent properties, not least of which is breeding... Cultivars aren't a patch on their heritage ancestors for medicinal qualities, for example. If one variety yields no benefit try another, I guess.
Did the vet tell you why they were euthanized? You said they didn't find anything viral or whatever, but strange, the birds must have been in a sad state for the vet to put them down. I mince raw garlic and let them eat it as they will or mix it with their food. probably mixing with their food is best to start with so if they don't want to eat the strange new food outright they'll get the taste for it which often gets them eating it.
What's the chances you have asbestos or fibreglass splinters around the place? I've found it in chicken coop floors, walls etc before. I'm sort of thinking it sounds more like a physical irritant than a viral infection, especially that part about the chicks rubbing their faces in the bedding. If I were you I would purchase a dust mask and use it. There's every chance what's irritating them is also getting into your lungs, but you're so much bigger you could inhale sizable quantities without getting more than a bit of a cough a little way down the track. If it's asbestos or similar, best to be safe. Have you told your vet about the chicks a few days old rubbing their faces? I hardly think you've got a whole flock of pollen-sensitive chooks, though it's not impossible. I think instead of viruses the vet needs to check for physical structures or something, just to be sure.... Or if the vet's billing you out of house and home, and if you've got the resources, maybe temporarily you could remove the birds to another location? Your coop may be to blame. I don't know, trying to suggest something helpful. Best wishes.
