Sorry to hear about your chicks Becca. My very first chick to get it was smaller than the rest and I think her immune system was compromised from the beginning which is why she got sick so much earlier than the others. It could be that your sick chicken in a Marek's carrier, or perhaps there is Marek's on your property from the previous owner. The virus can stay dormant in coops and soil for seven years.
If the hen got better with your care, I am doubtful that it was Marek's that caused her inability to walk. I've never had a chicken recover and regain its ability to walk (or breathe or see or whatever the case may be).
I think you will need to cull the little one just to be fair to her. However, it will not save the rest of the chicks. If the sick one has Marek's, then the others have already been exposed. It is too late to isolate them or vaccinate since they've already been exposed to the virus. Sorry for the bad news. If you are lucky the strain of the virus will be low virulence and not all of your chicks will develop the disease.
Thank you so much for your help! I'm hoping it's just something that shows as Marek's but will actually get better 😞 of course she was my favourite in the flock! Always the way 😭