I think I would try direct dosing with Vitamin E instead of depending on them eating the Sunflower Seeds.I started thinking it's a vitamin e deficiency so I've increased the sunflower seeds (chopped up) available for them. I've added ACV to their water too and increased their access to brewers yeast and kelp.
If my search and calculations are correct, (depending on where you look and how it's served/shelled/not shelled/type of seed, etc.), 1/2 to 2/3 cup sunflower seeds deliver around 35mg of Vitamin E.
Recommended therapy treatment for neurological symptoms like Wry Neck, etc., in poultry is 400IU (180mg) Vitamin E. So...you may be coming up very short in getting enough Vitamin E into the chicks to see if there's a real difference or not.
Might be worth a try grabbing a bottle of Vitamin E and direct dosing the chick.
Just a thought.
Sorry about the chick, I agree, it's very heartbreaking.
They have grit available, correct? And you have checked inside the beak to make sure there's no obstructions, hair in the throat, etc.