Chickens mask sickness by standing still, moving very slowly, or sitting safely under something. Flocks, in general, don't tolerate perceived weakness and will begin to peck at sick members or try to drive them out. Thus, sick chickens will hunch their shoulders and fluff their feathers as a sign of pain, but not much else.
If it is Marek's, isolating her won't protect your flock, as they have all most likely been exposed already. That said, there are still many other things that could be affecting your girl. When you get the chance, take advantage of a chance to worm her - and maybe the rest of your flock at the same time since this isn't really a great laying season anyway. Safeguard isn't very expensive, and it's really a great part of a healthy flock maintenance routine.
Also, having nutridrench available will come in very handy as long as you have a flock. I pretty much use it as my first line of defense for anyone who looks a little under the weather.
I also understand that life is not centered around chickens
and you have to do what is best for you.
In my own experience, I often do not take much time treating birds who have stopped scratching around or eating. I know this is hotly debated, but I cull them before more serious symptoms arise. I also know that if I'm finally seeing they are in pain, then they have been in pain a lot longer because they're so good at hiding it from us.
Wishing you the best with your girl, and so sorry she is not well.
