Sorry I can’t advise you either about the best strategy with young chicks.
Day 23 is a real possibility nothing will hatch with bantam eggs imo.
Just a few thoughts from someone who knows not enough to be sure what to do you or Mary:
To buy 2 or 3 chicks today is not a bad idea if you dont mind raising them yourself in a brooder if Mary doesn’t accept them.
I think its not a bad idea to candle the eggs before Mary wakes up to make a final decision. I know people do this with eggs in incubators, and you can do it too with sleepy broodies in the night.
I don’t have any idea what she will do if you add the chicks and leave the eggs in the nest too. Maybe she gets confused? And because the chicks are a bit older, Mary will leave the nest soon anyway after you added the chicks in the nest.
Do you have an incubator in case there are chicks inside who might be alive? If you start to warm it a few hours before you switch the eggs with the chicks maybe a miracle happens and Mary has 1 or 2 chicks she has been brooding too.
Adding another chick (or 2) in the night, who is strong enough to walk around with Mary (1 or 2 days after hatch) might work.
Good luck with whatever you decide to do. We all know you try to do the right thing .
Edited an ugly typo.