The progression of the lameness and subsequent paralysis points straight to an avian virus. I've been this route before.
https://www.backyardchickens.com/th...update-now-another-pullet-going-lame.1432738/ It ends in heartache.
With my third pullet that came down with these symptoms, when she reached the stage where she had declined to the point where she could not move much and had quit eating, I made her comfortable in a well padded box and drove her to my state animal testing lab. They took her and euthanized her and did a necropsy to determine what this was that was paralyzing my pullets. It was avian leucosis. Marek's is very close in symptoms, so it's usually one or the other that causes young chickens to go lame.
Give us your location, and we can help you find the lab closest to where you live.