If you have Marek's, you can't make it go away. You can assume all of them are exposed, culling isn't going to change anything, unless you want to end suffering. All your birds won't die. You can't really tell which ones will. but if they get over a year old, there's a better chance they'll survive. I know it's keeping you awake, but there's not really anything you can do about the past, and from now on new chicks will have to be vaccinated day 1 and quarantined for at least 3 weeks.
With the one that can't stand, I would try a multi B vitamin, animal or human, I would also make sure she's not dehydrated because they can be too weak to move if she is.
Sorry to hear about your hen. We've all been in your shoes and there's nothing you can do to go back in time. Don't beat yourself up too much. My Marek's came from a hen we rescued last year because I just had to have her. She's doing great, but I lost two full batches of chicks since then in addition to some other random birds that were older.
My experience is that once they get the paralysis they do not get better. My first bird was very gradual paralysis. Started out with laying around a lot, then a couple of weeks later she was limping, then she couldn't use one leg at all, and by the end she couldn't stand at all. This took four months until I finally sent her in for necropsy.
I don't want to make a decision for you, but the only thing I regret is that I kept her around for so long because I was in denial and during this time she infected my entire flock, my babies, and my house and barn. I think if I would have culled sooner I could have spared some of my chicks, but by the time I knew what was happening it was too late.
You can help us out by telling us what kind of chickens you have and how old they are. You should also be weighing your sick bird at least twice a week. If you see she is losing weight that is another sign that is not good. If she is eating and maintaining her weight and seems content, then it is up to you how long to wait.
Keep the questions coming! Also, I recommend reading every post of this thread (all 339 pages LOL). By the time you are done you will feel better knowing that you are not alone and that we all have moved forward with the disease. It is horrible to see your birds get sick and know you can't do much for them, but in the end we have to be good patrons to our girls and do what is in their best interest.