DE is not effective as a mite or lice treatment, but some use it especially in orther countries where pesticides are not available.
Whichever treatment used, the coop bedding and nests must be removed and burned or bagged, and replaced with clean new bedding after the coop is treated.
Permethrin 10 concentrate to be mixed with water is very good for both mites and lice on the body. Treatment is required once and again at 7 days for mites and 10 days for lice, to treat the bugs, and later the newly hatched bugs before they lay eggs. It can be used in a bottle sprayer or garden sprayer. Permethrin garden dust is also good to use, especially in cold weather.
Elector PCP is very good for treatment, although it costs a lot of money.
Ivermectin pour-on or oral is good for lice and mites, but requires a long egg withdrawal time. Dosage is 0.1 ml for each 2 pounds of weight. Ivermectin is excellent for leg mites, since leg mites require a different strategy.
Sevin dust (carbaryl is no longer approved for poultry, but has been used in the past, or now in some countries.)
Here are some very good articles with pictures:
https://the-chicken-chick.com/poultry-lice-and-mites-identification/
https://anrcatalog.ucanr.edu/pdf/8162.pdf
Scaly leg mites are treated differently. Ivermectin works well, and legs can be soaked once a week in warm soapy water. Use an old toothbrush to very gently loosen scales and any debris. Then apply a thick oil to the legs once a week rubbling it into the scales. Vaseline, coconut oil, castor oil, mineral oil, or vegetable oil are very good. Repeat this for at least a month.