Update:
We just put poor Big Red down today after she seemed to improve and actually ate for a few days.
I should have done it sooner but this was my first time and she seemed to get better and gave me some false hope.
She started out with being last to come out of the coop and first to go in. Stopped accepting mating attempts by Cleatus and he pretty much left her alone until she became weaker, then started trying occasionally

. Her comb got a bit purple at the back and she stopped laying. I discovered that her crop was like a walnut on a couple mornings. Coconut oil and massage addressed the crop but she still had next to no appetite and poop that was small, infrequent, and mostly like liquid urates with the tiniest little green pellet type solids. Urate material started to become yellow. Abdominal swelling would be up and down and then was not bad/maybe normal?, for a little while. We tried torpedo feeding for a few days but decided that I would stop all force feeding as she was probably not going to make it for long. We brought her in to be alone when she would fly to the top roost in the run to sleep instead of with Cleatus and Nuz in the coop.
She began to have abdominal swelling getting worse with the return of her appetite and thirst. Her comb also became red again without a hint of purple during the time we nursed her inside. She was alert more often and poops returned to almost normal. She was still weak.
I suspect reproductive issues like sudden internal laying but possibly brought on by Mareks. She never laid an egg that wasn't perfect but just stopped after 2 days in a row trying and failing to lay. Her comb never got pale even with her suspected stress molt.
We will perform a necropsy on her tomorrow because it was just too much for my daughter and I to do it right after ending her life.
I miss her voice that I haven't heard for a week. I miss my greedy bird that refused to give another chicken a chance to get a treat. She wasn't cuddly and didn't even like to be petted but she was still bold and friendly. She was my best layer and Miss Perfect. The strongest one and not a feather out of place until this just hit her.
I am down to 2 chickens from my original 5 and one is a rooster.
This is hard and I just had to get it out.