Depends on the breed of duck whether or not she will possibly incubate the eggs, white pekin probably not, muscovy, very likely. In my experience eggs that have been frozen will crack and of course be no good. If the nest is not full of down that the mother has pulled from her breast to line the nest my guess is she has no intention of incubating them.

I have more experience with geese, and they seem to wait until they have "enough" before they start sitting. The freezing temps are a concern, tho. Not sure how to check if they'd been frozen or not, unless they are cracked. Maybe throw the cracked ones out and keep the rest? The worst that will happen is that they will just get broken later if they rot and make a stinky mother duck! This can/will happen even if there have been no freezing temps, as some eggs simply won't be viable for whatever reasons, and will break anyway. Part of the whole process, and the mothers are pretty good at cleaning up the bad eggs, eating the broken ones, etc.. and taking a bath. Make sure she has a good little tub/kiddie pool with fresh water to wash up in!