I don't think it's Marek's from what I can see. Do you have Marek's in your flock, or do you only think that because of the legs? At her age a reproductive problem is possible, or other cancer, or organ failure, those are not uncommon. Depending on what it is it can cause all kinds of symptoms, and if she's not eating, or digesting, then neuromuscular symptoms can happen due to nutritional deficiencies. It would be good to know the answers to the questions previously asked, is the crop emptying, have you given any vitamins, etc. I would also feel her abdomen, between the legs below the vent, see if there is any bloat, either water balloon like, or very firm. Compare with another bird if you are not sure. Also feel her keel bone, is it well muscled or is the bone very prominent? You definitely need to treat the mites ASAP, they will kill her.