Well, slowwhitedsm, you've officially been evicted from your own coop by a bunch of uninvited, eight-legged party crashers. That's a special kind of fowl play, and we feel your itchy, shower-obsessed pain. Our take is blunt: you're doing a lot of things right, but those mites are running a full-on rave in your roosts, and you need to change the music before they trash the place. The fact that you can't step inside without an immediate shower is a screaming red flag that the infestation is deeper than a few dustings can fix, and your birds' refusal to roost is their way of saying the mattress is full of bedbugs.
You've already tackled the obvious steps, deep cleaning, permethrin, diatomaceous earth, and upgrading the dust bath with wood ash. That's a solid start, and it's more than many folks do when they first spot the creepy-crawlies. But here's the cluck-tastic truth: mites are relentless little jerks. They hide in cracks, crevices, and the ends of roosts during the day, then come out at night to feast on your feathered friends. Since your birds are still itchy and one refuses to roost, the mites are likely still living in the coop structure itself, not just on the chickens. This is a war of attrition, and you need to go scorched-earth on their hiding spots. Treat every seam, every joint, and every corner of the coop with a permethrin spray (when the birds are locked out and it's dry), then repeat the treatment in a week to catch the newly hatched eggs. And for the love of all things egg-citing, stop worrying about the cold spray on your birds, Poultry Protector is fine in 30-degree weather if you apply it in the morning and let them dry before nightfall. A cold, wet hen is a problem; a dry, mite-free hen is a happy one.
This whole saga reminds us of another reader's ordeal with a My senior hen's got a torn comb and I'm clucking worried situation, both are about visible symptoms that scream "something's wrong," but the root cause is hidden. In your case, the mites are the invisible enemy, and they'll keep partying until you crash their bash with a systematic, repeated assault. Don't just dust once and hope; mites can survive for weeks without a meal, so you have to outlast them. Also, consider that your birds' dust bath upgrade is great, but if they're too itchy to use it, it's not doing much. Add a shallow pan of sand and ash right inside the coop run so they can roll in it whenever they want.
Here's your concrete takeaway: you need to treat the coop and the birds on the same day, every five to seven days, for three full cycles. That means a full-coop spray (every nook and cranny) plus a dusting of the birds with permethrin powder (avoiding their faces) on a warm afternoon. After the third treatment, if you're still jumping in the shower, you might have a different species of mite, like the northern fowl mite, that lives on the birds full-time. In that case, you'll need a vet-prescribed ivermectin treatment for the flock. But start with the three-cycle blitz. Your birds' refusal to roost is the canary in the coal mine here; if they're that uncomfortable, the mites are winning. Turn the tables, and reclaim your coop.