DE does not work for many of us. I would just get some Permethrin 10 concentrate, either Martins or Gordons brand from TSC or other feed stores. Dilute it with water 37 ml per gallon of water, and use it with a spray bottle or garden sprayer for the coop. Use at least twice at 7 day intervals until the mites are gone. Empty all the bedding and nests, and treat coop, roost, and nests.
If you post some pictures of your coop we might be able to help with suggestions. Try to divert water away from your coop. Is the run covered?