We dusted all three of our hens with Sevin yesterday and they were NOT happy. Because we've only had them for about 14 days now, they aren't comfortable with us yet. I've been working on getting them to trust me but that's probably blown after the dusting yesterday. On the bright side, I don't have mites or lice because I was thoroughly dusted at the same time.
We worked some more on the big clump on Scrappy. I've gotten about a third of it off of her now. I can see that this has been on her for a good long while as the center main part is somewhat attached to her skin. On the edges of the worst part her skin is very raw and it looks like if I pull on it she'll bleed. So, we're slowing working on it daily. I do see smaller "normal" deposits of eggs at the base of other feathers, though. Do I have to get every single egg off of her? I might as well shave the poor girl.
I went to buy some Eprinex yesterday but our best feed store in town didn't have it. The guy there suggested Ivermectin Pour-On instead and I bought it. But, now I'm having second thoughts about using it. It's not the same formula as the Eprinex and I'm concerned about how long it will stay in their system and if their eggs would be okay to eat after 2 weeks or not. Do you think I should return it and get the Eprinex online?
As much drama as this is, I'm super happy that the girls are adapting to being here. They've never slept on roosts before and last night they actually used the roosts.

Made me so proud. I have an egg so I think they're feeling more at home. No more bullying happening and they greet me when I go outside. This is so fun!!!!
So, my plan of action is...
1) Work on the mass daily until its all removed.
2) Use a pour-on parasiticide such as Eprinex (or Ivermectin) and at the same time thoroughly clean and dust their entire coop.
3) Dust again with Sevin in another week to get any mites that recently hatched off of them.
4) Twice a year treat with Sevin and Eprinex.
We have DE dusted around their coop and run and I made a dust bath that has some in it although they haven't figured out what that's for yet. Will the DE kill any mites that hatch as well?
Thank you so much for all your advise, I don't know what I would've done without it.
5Leepy!