Her posture very much lines up with the Marek's I experienced with my rooster. HOWEVER... I am not saying it is Marek's based on that alone. I'm just saying that this posture and behavior is heart-breakingly familiar.
All I can tell you are my experiences. I do not claim to be an expert nor again am I saying that this is what your chicken has.
His physical symptoms were:
-Inability to move legs voluntarily. He rested back on his ankles or fully down on his keel. He often leaned back a bit. We had to form a "doughnut" around him with towels to keep him from rolling backward and onto his side (the laundry basket is brilliant!). Despite the towels, when he had to pass a poo, he would attempt to stand. He had the power but no motor control, and would throw himself backward and sideways. He could not recover from this position without our help. Once his heart started to go (from Marek's lymphomatic tumors growing into it) this caused him immense distress and he couldn't breathe. We had to tend to him at all times.
-As his diet decreased, at one point he would only eat mealworms, and then nothing at all. I tube fed him a few times. Despite getting the correct hydration and "gruel" fed to him this way, his poos became more sickly over time, turning more green. They were eventually quite a bright green.
-He was alert, very friendly, and bright-eyed the whole time, right until the moment we had to put him down because he could no longer breathe/get enough blood circulation (heart tumors). Very curious, tracked out movements, even looked at food-- just wouldn't take any. He was very quiet and would only make a noise when he'd tumble over.. a very quiet little cluck one or twice as if he were asking for help (which we obviously were tending to him at all times).
Even on the two hour ride to the vet to have his final evaluation he was bright eyed and made tiny mutters every time he saw a hawk or crow fly over the car.
I don't know if his wings were similarly without motor control-- he didn't attempt to use them in the final days.
Strangely enough, he had one droopy eyelid like your hen (no pupil changes), but he had that since day one. I don't think it was related to the Marek's. It was enough for the vet to ask me about it, though.
We noticed his lowered appetite on a Saturday (before this he seemed 110% healthy and vigorous)... Monday he went to the vet (and had surgery Tuesday), Wednesday he developed the paralysis, and by Friday he was very sick indeed. He went downhill very rapidly.
So, he went from "fine" to "paralyzed" in 4 days, and from fine to knowing his quality of life was gone in 7. When he was struggling so very hard to breathe and the diagnosis of lymphoma came in we felt it was necessary to put him at peace. I honestly don't know how long he would have lived had we not, but the vet was grim, and I know avian physiology well enough to know that he was dying.
If he wasn't struggling so badly for breath (and I got the diagnosis of lymphoma from the diagnostics lab while at the vet's office), I would have absolutely continued to try to save him the way you are with your hen.
