In your first pics, there could possibly be a tapeworm in the poop in the 3rd picture. A vet could rule that out, along with other worms and coccidiosis which can affect chickens with poor immunity. Tapeworms are normally treated with a wormer with praziquantel, but other worms are better treated with Valbazen or SafeGuard.
I would suspect Mareks disease, since she is lame and extending one leg. Chickens with Mareks may suffer a variety of common illnesses, due to a lack of white blood cells and thus, immunity. Here is some reading about Mareks:
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/