Don't be stressed. When chicks arrive, mama hen will accept them. I do understand your concern, about the hen that has already been sitting quite some time, and needs to sit longer than usual. I think broody hens are patient, and will do the extra miles. I also think that broody hens are aware of their eggs, as viable baby chicks in the making. They should not abandon ship, after 21 days. I suggest you provide easy access feed for them, so they don't give up due to getting hungry and weak. Water also, and within easy reach.
One possible option, is to switch out the eggs, not chickens into their original nests. Seems that the dominant chicken chose the nest that she preferred.
Is the number of eggs approximately equal under each hen???
You would need to do the swap quickly, and best in the dark.
WISHING YOU BEST,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
