I bought two ducks and had to switch to flock raiser by Purina.
Evidently the Purina is not that great because I have a 4 month old chicken with the beginnings of wry neck.
So I read to feed foods with Vit E and that seems the easiest...but I wasn't sure they could crack the seeds?
If I am answering your question in your headline, Yes, chickens can eat sunflower seeds. Particularly Black oil sunflower seeds. The are high in protein and the hulls are thinner. Wry neck is a neurological symptom that can be hereditary, or caused by a head or neck injury vitamin E deficiency, if the feed is old or in infectious diseases.
Just some additional info: Sometimes, wry neck and leg issues are associated with Mareks disease that does not show up until a chicken is a few months old.
You can make her a scrambled egg right now with some of her chick/starter feed in it, egg has the Selenium in it.
Also offer her Nutri-Drench, Hydro-Hen or Sav-A-Chick to give her a boost tonight. I know about giving her Vit E but, I do not know the dosing.
Hopefully an expert will pipe in about more treatments and to dose the Vit E.
I know Vit E is based on weight. So, get your kitchen scale out on a counter, turn off the lights. have her stand on it and get her weight. This is a trick I learned here on BYC. Well, everything I learn is from the experts on here.
Hope this helps you tonight.