That looks like a red mite infestation to me. Red mite live in the coop and not on the bird. Put all your efforts into getting the coop clean. Don't worry as much about the birds although treating them as well is a good idea.
While Ivermectin does still work and will kill mites it has two major drawbacks and one is in order for it to work the mite has to bite the chicken and suck some blood. The other is it works out very expensive. With a serious infestation you don't want the mites to bite at all. Ideally you want to kill the mites
before they bite!
Add to this that with a serious infestation the mites hatch in the coop faster than they feed so you never get rid of them.
Permethrin does work. But, it's a contact killer. The chemical has to come into contact with the mite. To kill all the mites, every mite in the coop has to come into contact with some permethrin.
What permethrin doesn't do is kill the eggs so you need to treat the coop over and over again.
If you have a large coop with lots of construction joints and open end grain making sure the permetrin gets every where is difficult. From what I can see in the pictures you've got lots of places where mites can hide.
The nuclear option is to move all the chickens to a new coop and burn the old one.
The next option is to use a blowtorch. This is the most effective method provided the coop is suitable. The coop in the picture below is suitable for blowtorch cleaning because it has few joints and endgrain and the risk of setting the coop on fire is low.
View attachment 3268906
In your case I would move all the birds out of the coop and spray with permethrin, lots of permethrin! You need to get the permethrin everywhere.
You need to repeat this spraying every few days until you can no longer see any signs of mite activity. I'm doing a small coop currently. I'm on the fourth treatment now and I had to dismantle some of the coop to ensure the spray made contact with the mites in the joints and move the birds out while I deal with the problem.
