Your description sounds like a thiamin/ribofavin issue. Thiamine/riboflavin deficiencies cause curling toes and lameness - and also sitting with the legs out like you describe. If that's what is causing the problem, (which it may not be!) here are some thoughts:
If you are feeding medicated feed, I recommend you stop it immediately. The medication is a thiamine blocker and some chicks can't take it. If not feeding medicated, that's good.
I think I read that you have already given the poly-vi-sol. Keep doing that.
Can feed some RAW liver from a good, healthy source (not supermarket - preferably grass-fed). chop into tiny pieces (or put through a grinder...slimy!) and give as much as she'll eat. Don't leave it out- just give what it will eat in one setting and remove leftover within 10 minutes. This should be fed RAW. Offer the liver several times throughout the day.
Good quality brewer's yeast also has the vitamins that will be helpful. Lewis Labs Brewers yeast is the only one on the market that's not grown on a GMO substrate (they can call it non-gmo because the yeast itself if not gmo, but most the other brands are grown on gmo substrate even if marked non-gmo). I have put this out in a dish and my birds will eat it right out of the dish. They love the stuff. Just be sure there is plenty of water out for them as it is quite dry!
I did stop the medicated feed when she got sick. I will try the raw liver and the lewis labs brewers yeast. I will try anything at this point! Thanks