Hi Janet, We feed a pair of wild ducks (closely related to mallards) in our yard too; they are gorgeous, aren't they? I love their chilled- out natures, and their little quack sounds.
They visit us each year just before she incubates her eggs, which can be 2-3 times in the breeding season depending on the success of her broods (sadly, her entire ist and 2nd broods fail to make it sometimes).
Mallards lay one egg at a time, cover it up, then return the next day to lay another one until she has 12-16 eggs. During that time she will be very hungry & thirsty. After all the eggs are laid, she will disappear for about a month to incubate the eggs and raise her hatchlings.
I'm sorry to hear that her foot is hurt. Its the kind of injury she can survive on her own by the sounds of things, but she could wind up with a wonky leg. If you're really worried, you could talk to a rehabber about it. I agree with you that its a tough call because she might have started laying eggs. But on the other hand these eggs will not hatch on their own witout incubation, so its not as cruel as taking a mother away from a nest with live babies in it.
Personally, a few of my favourite wild birds get leg injuries each year just in the normal course of breeding season. Some look like breaks, and other seem more like sprains. They always make it though.