I'm sorry that you lost the hen. I'm glad you are having the necropsy, though and, thanks for your willingness to share the results. It is very helpful to us all.
I would have suggested bleaching your coop instead of just using water. Marek's is a tough bugger to get rid of.
I do not believe this is fixable...not at her age. I am more leaning towards a neurological problem or Marek's. Marek's does not always leave them totally paralyzed. You can try this vitamin regimen for two weeks. If it doesn't help, I would humanly euthanize and have a necropsy done...
The one leg, out front, totally useless is the most known symptom of Marek's. She probably will not recover. Humanly euthaniziing and having a necropsy performed is the way to go. That way, you will know what you are dealing with. If it's Marek's, your entire flock is in danger. You need to...