I cannot speak from experience, as I think duck eggs are a little less finicky than chicken eggs. But one thing to keep in mind, is that if your hygrometer reads 1 or 2 degrees lower, you have actually lost considerably more humidity than that--it can't measure & adjust as quickly as the humidity can change.
You're right about the momma hen, but remember that you have already messed up the "natural" order of things by sticking everything in an electric incubator, and so you can't necessarily expect that what is okay for the momma is also okay for the incubator. After all, the momma doesn't need pans of water to regulate humidity, and she doesn't need ventilation holes. Her humidity requirements are different too, because under a hen the air is not moving--and therefore wicking moisture--as quickly as it is in a heated incubator, with air passing in and out very quickly in response to the temp differential between outside the incubator and inside (and aided by a fan if you have a circulated model). Consider the difference in your personal moisture level at 95 degrees--with and without a fan blowing. Are you dryer when it's blowing? You bet.
So I tend to trust the many voices on this board who say opening the incubator is not a good thing during those last few days. It can suck the moisture right out of the egg membrane and shrink wrap the chick inside the egg. I don't think this *always* happens, but it certainly can happen and is a risk to consider.
That having been said, you are not the first person on earth (or even on this board) to have a staggered hatch, and obviously we all open our incubators occasionally to deal with one issue or another. So you are probably not ruining the hatch by opening it. But you might have a lower hatch rate, so it's best if you minimize the opening as much as you feasibly can.
Regardless of all that--GOOD LUCK! Hatching is so much fun!
