Oh, she's eating and drinking. She's likely only off that nest 15-25 minutes a day, so unless you have a camera on her you're not going to catch it. My broodies get off the nest, eat and drink, take a huge poop, a quick dust bath, then spend a few minutes terrorizing the rest of the flock--I think taking out their frustrations!--then back to the nest. You're correct in that nature has it all under control!
I would advise to check how many eggs are under her and mark them so you can pull out any that are added. Once a hen starts setting, all the eggs start developing and should hatch within a 48 hour window. If eggs get added later, the embryo inside will only be partially developed when the rest of the clutch is hatching, and you'll have a non hatching egg. I just make a circle around mine with a sharpie, that way I don't have to move the egg to see if it's marked. I just raise her rear end up each night and do a quick egg count. If any have been added, I pull them for our eating eggs.
