Sorry to jump in here- I don't have anything of real value to add.
I've never delt with Marek's- I have vaccinated chicks - knock on wood-
But- aren't there sometimes changes in the pupil that are signs of the disease? Lack of response to light, grey spots, etc...?
I was just wonder, Little Miss, if you have tried shining a flashlight and looking at her eyes to see if they look normal and react to light?
Marek's was the first thing I thought too, based on other pictures I've seen, but I would give the St. Johns Wort and Poly-Vi-Sol (without iron) a shot, for sure. It certainly can't hurt at this point.
If she does have to be put down...a necropsy would be very important in identifying the problem.
Good Luck!