At the moment, I took the egg away so it does not end up getting smashed. But I cannot tell how far along it is compared to the one in my incubator, so I do not want to stick it in the incubator with the one that should be hatching soon.
my Muscovy is also nesting and sitting on some eggs that she laid. But they are not going to hatch, at all, because they were never fertilized. I would put the egg under her, but I'm afraid that she might end up smashing it since is it not one of her own.
For the egg you took away from the Mallard mother, I would pop it in the incubator while you think. It will not hurt the about-to-hatch eggs if it just sits in there, and that will keep it warm and safe.
Then I would recommend you candle the eggs the Muscovy is sitting on. You think they are infertile, but I would recommend that you make sure. (If any of them ARE developing, you do not want to give her an egg that will hatch at a different time.)
If you decide to give the Mallard egg to the Muscovy, she will never know that it's a different egg. Most birds are bad at recognizing eggs. Whether she will crush the egg, I do not know, but I'm sure she will not recognize it as an intruder.
I'm not sure if the Mallard egg would be safest under its own mother (father excluded), or under the Muscovy (with her own eggs removed), or in the incubator. I would probably trust the incubator more than either female duck, just because incubators do not crush eggs, but I'm not sure if that is actually the best choice.