I can't find any chick feed in that link... If you are using the shook chow, that's definitely an issue. I couldn't open the nutrition info because I recently overloaded the memory on my device. But as far as I can tell it only has 16% protein. Which feathers are made from 90% protein and it's amino acids. And to me Silkie's are very heavily feathered. SO I hate to focus on this too much. But if that is indeed the feed could very well be the root cause of your problem.
70 and rain... cocci can cause balance symptoms and is in every single chicken poo. If you want them to sleep in the coop you may need to train them to do so.
It may or may not be Marek's. But if it is, doesn't have to be a death sentence for your whole flock. The head hanging seems more on par with vitamin deficit. But so many things can look the same early on. My experience, was 2 lost out of 82 birds at the time.
How is her appetite? Is she drinking? Dehydration will cause major issues fast, so it's important to make sure she is. Many ill birds won't have much desire to eat or drink so we have to encourage them and drinking is the MOST important. Oh I realize maybe these questions are what you referred to as "everything is normal..
I would offer scrambled egg for the included nutrients and ease of digestion.
