A setting hen isn't laying eggs so her comb and wattles will likely pale and shrink. That is normally the case for any bird not in production.
The chick starter won't hurt the hy-line flock as long as they have oyster shell or other calcium source in a separate container.
At any rate, the chicks...
Perhaps she detects that it may still hatch. Usually they will abandon a nest when several hatch.
A mother hen is a formidable foe and will protect the chicks from all comers. IMO, they are better off with the flock. If you separate them for too long, introducing the chicks will be harder. By...
Are there more unhatched eggs in the nest?
If you are feeding layer feed, you'll have to switch the whole flock over to starter feed for a while.
Other than providing feed appropriate for growing bodies, the hen will take care of everything else.