I see this is a long-lasting thread, but I've also had a long-lasting mite issue, so will write a loooong post. We've had chickens in tidewater, VA, for 15+ years and have dealt with northern fowl mites (NFM) a few times before--classic loads of them around the vent, the dark icky mess around the vent, scabs forming, etc. Successfully got rid of them.
HOWEVER, last October, just after adding a new bag of bedding to the coop, we started getting bitten by near-invisible creatures, and saw under a hand lens that they were mites crawling on us when we scooped poop daily. But, like the original poster here, these were in large numbers in the coop, gathering on top of the nest boxes, on top of the roosts, NOT nocturnal, and relatively few actually on the chickens compared to our past experiences with NFM--none of the telltale black goo around the vent. They definitely were not red/roost mites.
I became suspicious that they were tropical fowl mites (TFM), since I read they lay are more apt to lay eggs in the bedding instead of on the bird, and my bedding came from a southern state. My vet looked at a sample (she thought from my description they didn't behave like NFM), and said they weren't red mites, but the difference between NFM and TFM is very hard for a non-mite expert to tell (a few hairs visible under a microscope).
So I sent samples to my Cooperative Extension for ID, and all they came back with was "mites," and sent me the usual info on NFM. They didn't have someone who could tell the difference between NFM and TFM. I finally found a mite expert at the University of Florida, and sent them a sample. They came back, much to my surprise, as northern fowl mites! I'm following up with them on why mine are behaving so differently.
But I feel your pain (and itchiness). We've tried oh-so-many things-- changing the bedding every few weeks, Elector, permethrin, DE, sulphur, topical Ivermectin...Something that seemed to keep the levels from exploding was careful observation on where they were congregating in the coop, and spraying those places with rubbing alcohol daily when the chickens weren't in there. They also gather on top of the bucket we scoop poop into daily so we alcohol spray that. They like to go up to some degree, so after we scoop with gloves on now, we walk back inside with our hands held up, so they crawl toward our fingers instead of up our arms and then wash the gloves and our rubber boots.
We also hung panty hose filled with sulphur from the pop door so the chickens brush against it and dust themselves twice a day. Didn't want to put a lot of sulphur in the coop-- to scary for the chickens to breathe that much.
One caveat is that our rooster seems to have more on him (like, a lot) than the hens do. He's hard-feathered, and doesn't spend as much time dusting, and I feel like he doesn't get the dust up under his feathers like the hens do, so it's possible he just keeps passing them along to the hens. It's also hard to get any treatment (spray, dust) next to his skin because of his sleek feathers. I gave him baths for the mites, and a gazillion came off (and crawled up my arm), but it was really challenging to get him all the way wet. Therefore, we treat him with Ivermectin pour-on in addition to the topical stuff.
Next to try is PremoGuard, and steam cleaning.
I feel like I could write a research paper on their behavior. At least we are avoiding getting eaten ourselves as much, now that we know how/when they're getting on us.
