I would set up a different place for the broody, thoroughly dusted with Sevin dust. Move the eggs to there, dust the broody thoroughly, or put Ivermec pour-on drops on her. That cleared my birds right up.
After you have her moved, start the war on the coop. Twice, you have to do everything again in two weeks.
I would definitely be worried about mites on the babies. It can weaken them if they get too infested. What day are you on for incubation? If after your best efforts you're still seeing mites, I would remove the eggs and hatch them in an incubator, then brood them well away from the coop to let them get age and size while you continue to work through the clearing it up.
I've had repeat issues with only using Sevin dust. Much better results using Ivormec on the birds, Sevin dust and DE in the bedding, and kicking the birds out while I muck out the coop and spraying permitheon (sp?) spray into the nooks and crannies. I then let the coop air out for a couple hours, put in new bedding, and dust it thoroughly with both Sevin and DE.
You need to get all the cracks, where the roost attaches to the wall, any seam between boards. Last year we ran a bead of caulk over most everything to avoid having the cracks and make it easier to sanitize.
I don't know where these bugs come from, but I'm not having it! Has to be wild birds, but they've been shut out from the runs since last July. Maybe they can be picked up in the grass when I let them out?
Since adding the drops of Ivormec to the chickens I haven't had a problem, 5 drops on a large sized hen, sort of like frontline for chickens! A drop on the back of the neck, one under each wing in that bald spot they have (it must be on the skin) a drop above the tail, and a drop below the vent. Two person job, one to hold the chicken and find skin, the other person applies the drops. We've been bug free for 3 months now.
