She can't be broody anymore. Period. Don't let her at or near the nest boxes for any amount of time for days, maybe a week. Weigh her daily to make sure she's not losing. Keep feeding everything she likes.
As for the neck thing, this is bugging me...I forget. Vitamins, electrolytes? Try making...
Awww...I've had lots of hens raise chicks together, it's really sweet.
Once these two hens hatched just one chick together. One hen would find something for it to eat, then the other, it was really funny! He grew super fast and always stuck close to his moms. :lol: