Layer feed would not cause her to have a thiamine deficiency.
Marek's doesn't automatically mean she can't use her wings and legs.
It also doesn't matter if this bird was never around the last bird...if it is marek's disease it's in your dirt....air...blows around your yard and it's every...
You just recently had a bird that you put down that has issues with balance/ and her legs.
You should think about getting a necropsy done, if you lose this bird, to see if you are indeed dealing with Marek's disease.
Maine
https://extension.umaine.edu/veterinarylab/