I am sorry for your loss, but she did look like she was suffering. I wish that you could have got some testing and a necropsy on her to have gotten a diagnosis.
Is there any way that you or a friend could put her down? Or ask a vet to euthanize? I don’t think that you will know if it is Mareks or not without the state vet doing a necropsy when she dies. It is possible that her vaccine was missed at the hatchery, or she may have been exposed in the 2...
Sorry that you are dealing with illness in the other animals. At 4 years old, I don’t think they had a bad hatch. Mareks symptoms usually show up much earlier in the first year of life, unless they have recently been exposed by a new bird in the flock who is a carrier. So many things can look...
If you lose another, send the body to your state vet in South Logan. That link is in this list:
https://www.metzerfarms.com/poultry-labs.html
I would inspect your feed for any mold or bad odor. It can mold quickly if any gets moisture in it. Mareks tends to cause lameness and curled toes in one...