Glad to know she is still gaining more energy! As far as her feathers looking scruffy/dried out, is she able to preen her feathers at all? If she isn't preening, combined with the fact she cannot currently dust bathe, that will lead to unkempt feathers. Also, is she eating on her own yet? If not, I would offer her something extra tasty to tempt her, such as scrambled eggs, canned cat food or tuna, mealworms etc. If she will eat that, you can mix into a mash until she is eating chicken feed on her own.
Re Mareks, have any other birds in her flock become paralyzed/unable to walk in the past? Birds paralyzed by Mareks often (not always) display a "scissored" look, with one leg straight out forward and the other leg straight back.
Have any chickens gone blind? If your answer is "Yes", then uh oh. There are some very informative articles about Mareks Disease here on BYC.
If your vet can see her in two weeks, as long as you don't mind caring for her and she isn't suffering, it is reasonable to continue care until then. If she shows no improvement in mobility by the day of her appt, you will need to trust your intuition as well as your vet's advice, whatever it may be. Either way, Ethel was in dire straits when you found her, and her immediate need was rehydration and nourishment. Whether or not she is able to recover, she has known your kindness and care, and she has responded positively.