Hi
I would agree that it is most likely Marek's.... Silkies are particularly susceptible to it. There are no hard and fast rules with the disease. My guess would be that the stress from being harassed by the young cockerel has triggered this second attack, rather than physical damage.
If she still has an appetite, then there is definitely hope. I only euthanize once they lose interest in food. That said, with limited vision, she is going to struggle to regain her confidence to go back into the flock even if she gets better from this, especially if she has been separated from them for some time, so you may have to make a difficult decision based on her potential quality of life as a single chicken living on her own even if she regains neck function. I find they do best with a little friend as companionship and competition for food sometimes motivates and stimulates them to re coordinate through physiotherapy. Finding a suitable sick bay companion is not easy and bear in mind that she will be shedding the MD virus if that is her diagnosis.... so you could be risking another bird to save her.
What are you currently feeding her apart from the supplements and what does her poop look like? You might want to consider fermenting her feed (chick crumb or grower), if you don't already, as this will help her digestive system to be more healthy and that in turn will enable her to get the most nutrients out of her food to support her immune system. A little egg once or twice a week and some canned fish in oil should also benefit her.
Companionship, sunshine and grass have been big factors in birds of mine that have recovered from Marek's attacks, but in the depths of winter, these are not things you can easily provide. If you can nurse her through until Spring grass/sunshine, you may see a miraculous recovery, but that is a long slog away at the moment. I have had the odd one that survived several months with supportive care through the winter and then recovered, so if you are OK with offering her support and she is still eating, then good luck to you both.
You might want to try a neck brace.... she will probably hate it but it may help.... some people have used the centre tube out of a toilet roll and snipped it with scissors at the bottom end to flare it out where it meets her chest and snip and fold down a notch in the front at the top for her chin to rest on. Don't persevere with it if it really stresses her and she doesn't settle as stress makes Marek's worse.
Best wishes
Barbara