Broody grey andulision
Personally, i don't interfere with momma - she generally knows what she's doing. I leave her free to come and go from the nest as and when she pleases, and let her do all the hard work.
Ah, i understand and good idea to keep her away from the flock. Yep, its a very long 21 days - seems more like 42!
Don't separate momma from her chicks - she will decide when she's had enough of them. If they hatch in the main coop, the little ones should be fine with the protection of mum, but you may need a separate area so they can access starter feed and water. I have used a small crate in the past to serve as their "feeding / sleeping quarters" (kept in the main coop) which i leave open during the day so mum can take her chicks out and about. We all do things differently and according to our circumstances.