I don't know for certain BC I don't know what you're fighting but I'm guessing lice. We've just had a fight with northern fowl mites and I think we've finally got them gone. However in my reading it seems to me that I was reading often if the nits have been killed they stay in the feather shafts until the feathers are molted. I'd do some more research before deciding to cull.
Yes, you are correct, I am fighting lice. I had asked someone on here that had battled lice & they had said that everything would be gone including the nits. However, I will do some more research as you suggested. That would be bad if I've been subjecting these 2 chickens to all these treatments if it was really already cleared up!