I agree it would be worth throwing some things at her unknown illness. I certainly would!Each morning it would be good to feel of her crop to make sure that it empties overnight. Some young chickens can have reproductive infections or disorders that may not be treatable. The way she stands is common for a sick chicken. When she was still laying in August, did you ever notice any shell-less eggs? When you have a sick hen with an unknown illness like this, and cannot see a vet, it might be good to worm her with the Valbazen, and perhaps give her an antibiotic such as Aqua Mox fish amoxicillin 125-250 mg twice a day for 10 days, in case of a reproductive infection.
I still think fluid will be critical for her survival. Keep at it.