You've got very good instincts so far! Well done (Moving the birds, treating the mama, hatching the last egg, waiting for chick to be walking, etc) To add the chick, I'd wait for a time when mama is calmly sitting & staying on chicks. (probably when they settle down just before dark) Sometimes a mama will peck at a new/different chick as an intruder, so if the mama doesn't see the chick she won't peck at it. Just slip the chick under her & she won't be able to "see" it. Chick will feel cozy & should stay put. When they wake up in the morning, the chick has already spent the night & will most likely be considered part of the family. It's best to check on the broody a few times in the beginning & again early in the morning.