Scaly Leg Mites are treated differently than lice and mites on the body. If you found clusters of eggs under the wings, likely that is lice and she needs to be treated with poultry dust or spray.
Sounds like she has had neurological symptoms most of her life. Could be that it's genetics, a developmental disorder or disease like Marek's. Hard to know unless you lose her and a necropsy is performed by your state lab.
I would still treat her with vitamin therapy to see if that makes a difference. Vitamin E to help with neurological issues, B-Complex for legs and well being and egg/tuna for the uptake of E.
She's only laid a few eggs in her lifetime - it could be related to her always being "off" or she may also have something wrong with her reproductive system (again, hard to know without necropsy).
She's kept from food - this will also contribute to a decline if she's not getting the nutrition she needs. Can you add another feed station or 2? I have a few hens that always liked to monitor the feed station, so I added more. Also in summer, I usually have another water station as well. Keeps squabbling down and everyone gets food/water.
