Just at a glance your mallard pair look like they are crosses. The hen looks like she could be a Harlequin cross and honestly, the drake could be, as well, since he has some frosting in his chest.
From what I know, both parents have to have the silver gene for the offspring to be silvers. If they are crosses and carry the gene then it would be possible for some of their ducklings to have the pattern.
As for ducks being monogamous, that is almost never the case in domestic ducks (some may pair up dutifully, but they are the exception to the rule). Out of all of the birds I've had, ducks seem the most intent on going after anything that moves. They may pair off and court, but if the boys think they have a chance with someone else they will rush for it.
I hope that helps. You won't really know for sure until the feathers come in, but the silver gene seems like a pretty safe bet.
EDIT: Just as an aside, it could be snowy _or_ silver. Both patterns are similar as ducklings and as adults. I'm familiar with harlequins, not so much with a mallard/snowy cross.
Also to answer your question about keeping pairs separated, the answer is absolutely. Anything in a pen together or free ranging together has a chance to cross. If you want to guarantee a breeding among some birds but not others, you have to pen them up separately. And birds that have been running together can produce offspring from a male for an average of 2-3 weeks, with cases of a month or longer.
