I'm dealing with the same issue so found this thread. We have had our girls separated all summer from the roosters in the hopes they could regrow their feathers in time for winter, but some girls just seem to be getting worse and worse, despite no mating, to the point where it's somewhat embarrassing.
However, I don't see how feather picking and mites are necessarily separate issues. If they're picking their feathers, they may be doing it because they are uncomfortable or in pain. If you do a simple google search on depluming mites for example, here's what you get "parasitic mites that burrow into the feather shafts and surrounding skin of chickens, causing intense itching and irritation that leads birds to pluck out their own feathers, resulting in broken and sparse plumage."
Also, when I look at your pictures, I do think I see some specks. I have a rooster that likes to get pets on my lap and I noticed tiny little specks, no larger than a pencil dot, moving up and down his legs occasionally. At first I thought perhaps it was scaly leg mites and that maybe they are more visible in some cases. But then I followed one of the dots all the way up and sure enough when I parted his tail feathers, I found several of these dots or specs hanging out around his vent.
