Sorry to hear about your sick chicks. It sounds like Marek's to me as well. My only advice, like the others, is to send your pullet in for necropsy to find our for sure. You'll have peace of mind afterwards and can move forward with out the "mystery" looming over everything.
Your older birds may be OK. That said, I did have one 11 month old hen that started limping slightly. I found her dead in the coop a few days later. No doubt in my mind that it was Marek's. I also had a 9 month old that we culled in December due to wry neck. We assumed she got into a toxin of some sort, but now I'm wondering if she didn't have a Marek's tumor in her brain or something.
I, too, had a beloved pullet that I treated in the house for over 2 months before sending her for necropsy. Now my whole house is infected with Marek's. I don't regret trying to save her initially, but I do regret not making the decision sooner to end her battle. I currently have three more sick pullets. Two I'm pretty sure are sick with Marek's and the third seems to have something else. The older one gets to spend one more day basking in spring sunshine before we cull her. The smaller one we will probably cull as soon as the healthier one is ready to rejoin the flock. When they have lost so much weight and have not energy to even walk outside for scratch grains, it it time to let them go.
You will still be able to get more chicks in the future, but make sure they are vaccinated and that you keep them isolated for at least 6 weeks when you get them. Marek's is horrible, but you'll learn to deal with it and move forward. It is devastating at first, but it gets a bit easier as time goes on.
I hope you get thru this quickly , because the time will come when the deaths slow down or stop. Sorry you're going thru it.