Ivermectin is the only one that I know of that will get rid of them. Let me see if I can find dosing info, I don't remember. Pretty sure there is an egg withdrawal, but you can wait till fall if you need to. Depluming mites are less serious in comparison to the other mites and lice.
I don't...
Yeah. Depluming mites live under the skin and cause irritation to the point that they scratch their own feathers off while preening.
It seems like there is a lot of mysterious feather loss in chickens it seems. I guess you can see what happens after molt, or maybe someone else will have some ideas.