You said mites: that's the start of the problem and must be addressed immediately as she will be anemic and die. The other issues are all stemming from this.
Kill the mites first: dust the bird with a permethrin dust - "Poultry dust" available at most
TSC and other feedstores.
Mites don't necessarily stay on the bird - they often actually live in the barn and come out to feed. So you won't see them on all your birds. You must thus treat the premises as well. Dust the bedding, the nest boxes, the roosts, etc. If you can, you'll want to dust the cracks of the walls. Some people make a "paint" of the permethrin and paint it on the wood to get in the cracks. That's where some mites lay their eggs - cracks of the coop, etc.
You will want to retreat twice - in five day intervals - on the bird. Treat all your birds.
Now you need to treat her anemia: make sure she is eating her complete feed mainly. If she doesn't, tempt her by wetting it so that if you grab a handful it sticks together, but if you drop it it falls apart. Make a small batch to give her first thing in the morning: In the water that you will use for this mixture, you'll mix 1 teaspoon of yogurt, about 2 teaspoons of water. Use that to moisten crumbles. (You can add boiled/mashed/wetted egg yolks to great effect.)
Also I would definitely give her vitamins for a week, then every other day for another week, then twice the next week. Enfamil non-iron formula PolyViSol baby vitamins will do and are easy to give. Three drops in the beak's side. You can find the non-iron at CVS as well as other stores.
She also needs to continue drinking; make sure she does so.
She absolutely MUST have her blood built back up, and the mites MUST be killed or she will decline horribly and quite likely die.
As for the DE, you can use in hopes of reducing numbers of future parasites. But as for mites, they're tricky to get and so easily brought into the barn by wild birds. It only takes 48 hours before their eggs hatch. They bloom and take everything down. That's why painting the interior of a coop to eliminate as many cracks is very useful. Then keep checking them every couple of weeks to make sure. Check at night: some mites only get on the bird at night to feed.
I hope this helps. Please feel free to ask any questions or email me or whatever you need.