Oh, I am so sorry to hear you are dealing with this! I know I became very frustrated and felt so bad for the birds! I did loose the two, and then one more of my Buckeyes succumbed to the same issue! The last one broke my heart! I kept her alive for at least two weeks, and she was tough through it all!
I believe that these birds were exposed to Mareks, and we're carriers when I purchased them. I bought them from someone who breeds chickens, and other animals, off of craiglist. I recall him claiming security, as he didn't want anyone walking around his farm, basically just brought the birds out in cages he wanted to show, or ones I had expressed interest in. I should have walked away at that point!.... Anyhow, I don't want to drag on...
He did not give them the vaccine, as do the hatcheries(many hatcheries do so automatically or will do so for a small fee at one day old) So, the few I had in my flock, actually had the Mareks vaccine, as sunnyside is an automatic(which I didn't even know, until this occurred!). Gosh, all the things you think you know, but don't!
Has your bird received any of the vaccines? How about the rest of your flock?
You were wise to keep her away from the others. Also, practice as much bio security as you can. Washing your hands in between caring for her, and the others. Disposable gloves used on her, making sure dander and what not are not on yourself, clothes, shoes, etc.
For us, I feel we have been contaminated as this is difficult to rid and can live for up to a few weeks, perhaps a year on surfaces!(I read various estimates-eek!)
So, my plan, is to always purchase one day olds that are vaccinated. If I hatch my own, I will purchase the vaccine and administer it myself to all our hatchlings. I won't put myself, or the sweet birds through any of that again, if I can avoid it!
I think I answered your questions, but if not let me know!
Keep me posted how it goes and what you do!
I hope your hen perks up and makes a full recovery for you! Maybe the vitamins are all she needs, who knows!