She may have slipped tendon (s).
As sad as it is, I think you should cull her. Her brain and GI tract works fine, and she will happily eat/drink/poop ect but as she grows, she will get sores on any part of her leg that touches the ground that is not supposed to. The bottom of the chicken foot is the part designed to weight bear, not her hocks or the sides of her feet. She is coping with your help, but it is really unlikely that she will be able to lead a close to normal life.
It is kind that you rescued her from wherever she was that let her get so dirty. She was probably getting walked all over by other chicks and having to drag her self through poop.