A (sad) update for those who'd like to know:
It did end up being Marek's, unfortunately. We took her to the local bird vet and she said she'd gotten several other cases of Marek's from the same farm we got them from as chicks. The farm that supposedly vaccinates all their chickens. They ended up putting her down in the same visit, which was really rough as she was my mother's favorite, but it was what was recommended and it seemed the kinder path to take.
Hopefully the rest of our flock dodges the symptoms and can stay healthy, but they are all certainly exposed to it now. (With this and a past tragic chick incident our life-to-death ratio with chickens is 3 to 12). I know its a number's game with chickens, but our numbers are especially bad, and if we end up loosing even more of them we'll probably call it quits. Hopefully the remaining three stay strong and healthy.
Would it be worth checking back with the farm we bought them from? When we first got this batch from them one 6-week old ended up dying not long after arrival at our house, we reached out to the farm and they called it just failure to thrive and gave us a refund. Could it have been Marek's back then? It wasn't exactly the same but she was flailing and falling a lot and tossing her head before she went. I know another refund probably isn't possible but we have put a lot of care and money into these birds, and just getting something back would be appreciated. (Just today the total vet bill here was $330 good god.)
My mother's out as I type this to put the rest of them in for the night, though they seem to be hesitant without their missing friend.
Regardless of how this ended, thank you all so much for your suggestions and well-wishes.
Wishing luck, and health, and long happy lives for all of y'all's chickens!