The boxes aren't that high. When momma is ready to leave the nest (usually 2-3 days after the first chick hatches) she'll just hop down and call the babies. They'll willingly tumble down after her, and that's not too much of a drop. You can pile some bedding under for a cushion of you want. My broody mommas usually don't go back to the nest box, they make a nest on the floor to brood the chicks.
You'd just need to be sure the feeder and waterer are low enough the littles can reach.
I agree that buff has a lot of eggs under her. did you set them all at once, or did she continue to add to her clutch after she started setting? If they were all set at once, they should all hatch around the same time frame. If they were added along the way, you can try putting the ones that don't hatch under the other hen. Thing is, though, if they're staggered like that, you might have chicks hatch in just a few days, and some not for another two weeks. You can candle the eggs that are left when the buff leaves the nest and see how far along they are and make your decisions then.
Regarding your new broody, a hen usually has to have been setting close to the 3 week mark to take chicks. There's a hormonal thing that happens while they set. Trying to give chicks to a hen that's only been broody a week or so doesn't always end well for the chicks.