Chickens with Marek’s show their symptoms in various ways. I don’t believe being able to hold her head up would be enough to dismiss Marek’s as a possibility. There’s no guarantee at all that the others will ever show symptoms. I suspect my own flock has Marek’s due to multiple unknown deaths and a case of paralysis in the past. However, my small flock has shown no symptoms for at least a year after the last death. The rooster I had that got paralyzed survived for months with Marek’s symptoms until he died.
Again, there’s no way to be certain unless you get a necropsy done on a deceased chicken. Though if you suspect your flock has Marek’s, you should not give away or sell any of your birds since you don’t want it to spread to other peoples’ flocks.
This article has given me a lot of information and will probably be helpful to you as well:
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/.