The poop picture looks like the normally white urates are coral or pink, which can be normal at times, especially overnight. If there is blood in the actual poop, then it never hurts to treat with Corid if you suspect coccidioisis. I recently treated a hen who lost balance for 5 weeks. She had good leg strength and usually could stand, but would fall backward or sideways after taking 2-3 steps. Then suddenly she got up and walked and has not stopped since, although she wobbles some.
So, it may be Mareks, but I feel like giving them a chance to get better, especailly since I have never seen Mareks before in my flock. Make sure that she can get to food and water. Think about using a sling if she will tolerate it. Try vitamins, and offer her usuall feed with some egg or tuna for extra protein.