If you can provide me with a link to some instructions on how to clip their wings safely, I would prefer that method.
The ducks would rarely be waiting for someone to come home, and when they do end up waiting, it would be no more than 2 hours at the most in their pen without diapers on. I wouldn't even consider getting ducks if we didn't have enough time to be with them now or in the distant future.
If this means you've decided against pinioning or at least leaning against it, bless you. I know some people do this, but I can't help thinking it's a cruel process. That's not related to only physical pain, but also to emotional distress. Pinioning is permanent. The wings do not grow back as the feathers do when you clip, and pinioning is not reversible by any means I've ever heard of. I just don't believe that the ability of flying birds to fly should be permanently disabled. If these birds ever go outside, which I assume they at least occasionally will, pinioning could be a death sentence because you've removed their best chance of escaping predators. If you're terribly concerned about flying and see pinioning as your only viable option, I strongly encourage you to opt for the Cayugas.
As far as wing clipping goes, there are many videos on the web, some better than others, that demonstrate it. If you do decide on the Mallards (Cayugas wouldn't need it), I'd also encourage you to make one visit to an avian vet with both your new birds so he/she can demonstrate on one, then supervise you doing it on the other. It isn't at all difficult if you know what you're doing, but you can hurt the bird if you don't. I've seen it done many times, but I've never clipped any of my own birds wings despite the fact that our female cockatoo used to fly across the street and perch in a particular tree in our neighbor's yard. Even though I'm pretty confident I could do it correctly, I'd still take a couple of my birds to the vet for supervision of my attempt, the same as I'm advising you to do.
I hope I'm not coming across as sanctimonious because it's really not my intention. I don't think you'd find many proponents of pinioning in this forum, for the reasons I mentioned above and others.
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https://www.backyardchickens.com/t/754458/mallard-thread-not-rouens-mallards is a long thread dedicated to Mallards in particular. You should take a look at that and ask your Mallard questions so you can find out what others who own them do about the flying. If you decide on Mallard house ducks, they're obviously not going to fly off into the wild blue yonder, but it's possible that they could flutter around your house and knock over vases and stuff. You just have to weigh the pros and cons.
Hope this helps, and best wishes for whatever decision you make.