Since she is an "older" production bird, it could be related to some type of internal laying/reproductive disorder. Peritonitis, Ascites, cancer or tumors can also affect the legs/mobility. Have you noticed any bloat or feeling of fullness in her abdomen? Have her eggs been "normal"?I suppose it's always possible. She's been acting weird off and on for months though, starting in October, and then for the whole month of December she was perfectly fine, or so I thought, then I found her collapsed out there this month. We are always battling Capillary worms so that is what I treated her for, but I don't think that is what it is either. If it's Marek's it's progressing very slowly. She's a Golden Comet and I always get the impression they were not bred for longivity especially, although we have another one out there who is six years old. Twinkie is only 3. From research,I've concluded that there's a jillion reasons why they can get leg pararlysis..it seems to be very common.
I hope you see improvement soon with the supportive care you are giving her.