Oh man, that give me a lump in my throat. It looks like a neuro problem. This is what I would look at. If she acts sick, I may think botulism. Look at the history of your flock. Is she new? Any others new? Possibly within the last 2 months? A carrier could stay healthy while making others ill. Have you thought about a bad peck on the head or even Marek's?
If she doesn't improve, you may want to think about euthanizing and sending her to a lab. At some point. You could stand her up in a sling in the meantime. I have made one out of a kitchen towel with 3 holes in it, and tied it to a pole or something, and place food at a reachable height. I can't make decisions for you, just try to help you.