I deffinately think now, after seeing the pictures that you are right about it being lice or mites.
O googled lice and mites on chickens and saw some similar feather damage like your pictures show.
As far as a remedys there are several, some of which you have probably tried.
First i wonder if you have a dust bath for your chickens? Dust bathing is a chickens natural defence gainst lice and mites. Some people like diacamatious earth others say it has no effect. I personally would set up a dry protected spot, i would use an old tire and mix fine sand with some of my wood ashes and see if they use it. You could even give them a bath in it by hand. The idea is to smother the bugs.
you have to not only treat the chickens but the entire coop also. All the wood and espically the perches. Some bugs hide in the wood grain and feed on the chickens at night while they are sleeping Some install only non wood perches for this reason. There are chemical sprays also orange oil is helpful to some.
It would be really helpfull if you could get right up close to your chickens skin with some magifying glasses and figure out exactly what kind of bug we are dealing with here. Sometimes they hide around the vent area.
If you could get a magnified picture of that and post it here, that would be great.
Also take some white paper towel and wipe it on the underside of the roosts and see what you find if anything.?
This has really got me curious.
Good luck and keep us updated!