First off, this is not true. Now, before I go any further, I will tell you how I know this.
For the last year, I have battled Marek's in my flock. To date, I have lost 35+ birds. If there is one disease I know, it is Marek's.
Marek's strikes in many ways...only a few of my birds, I can count them on one hand, have shown neck symtpoms. Some of them, have shown no symptoms...alive and running one minute, dead the next. Those, I had necropsied. They were rittled with tumors. Just none in the nerves that would have caused the classic paralysis.
If Buster is jumping on her, you need to remove her for her safety. If chickens sense something is wrong, they will jump and kill the sick/injured bird. If you want to do something, bring a friend inside with her. He also could have injured her sensitive brain tissue. As for the Marek's and separating...it has been shown that a bird with active Marek's sheds more virus then those that are asymptomatic...so, by leaving her in there, you are actually putting your other birds at a higher risk.
As for Stumpy...sounds like a birth defect if he has been that way all the time. Marek's does not show that type of symptoms...especially in very young birds. I have not lost any chicks before the age of 10 weeks.
My best advice to you...remove and quarantine her. If it's Marek's, she will not last long at her age. It will kill within a week, two at the most. Give her vitamin therapy..the vitamins in the water will do until you can find the poly vi sol. If she passes, have a necropsy performed so that you will know what she died from. This is the only way that you will ever know....and, for your sanity and the sake of your flock, this is truly the way to go.
Does she have any neurological symptoms on her neck? Paralysed neck is one of the typical symptoms of MD.