Please don't blame yourself. Your predator situation has changed. Unless you intend to stay outside 24 hours a day watching the ducks, you cannot possibly control what happens to them. I love when domestic ducks show up on the river, which is now rare since fortunately Easter ducklings are no longer considered good gifts. Their presence adds diversity to the wild duck population, and the idea possibly mating with mallards would badly affect the mallard population is totally ludicrous. Believe me, the last standing duck on the face of the earth will be a purebred mallard.
There is only one pekin now on the river where I am at, and he can as readily and quickly fly as a mallard! I've never seen anything like it. He's not as big as some pekins, but he is much larger than a mallard, so his mother could not have possibly been a mallard. Until now, I've only seen pekins make flying leaps downward, and quickly move across water by flapping their wings. This drake easily flies upward and across the river with the mallards.
The only domestic duck breed I've seen that can successfully reproduce in the wild; readily fly, including distances; and obviously outsmart many predators is the Muscovy duck. But no one should get Muscovy ducks in a residential area! They can rapidly multiply, because those little females have some uncanny ability to make nests where predators can't get their eggs; and they can perch on decks, piers, railings, rooftops, etc. Many people hate them with an absolute passion, including because many think the drakes are so ugly.