They need some source of warmth whilst they are feathering out because they can’t regulate their temperature without feathers. I know that you gradually decrease the heat in the brooder as they get older. I have always had a hen raise chicks so unfortunately I can’t comment on the specific temps...
Yeah so the crop is a muscular pouch that sits at the right side of their breast and when it is full of food should be easily felt and in chicks it’s pretty easy just to see it sticking out because they don’t have their feathers yet. They will eat and digest intermittently throughout the day and...
She may just be a late bloomer. If she is otherwise acting normal, I would just make sure her crop is full at bed time and emptying by the morning to make sure she is getting enough to eat.