I know they do get off - I've got/had four other broodies this season who've all got off to eat & poo. The difference is they go back to their eggs at some point. She will just go and brood anywhere, eggs seem to be fairly irrelevant so I have to check on her all the time to see if she's still on them. Anyhow, I will give her an egg or two back, I just don't want to risk too many because fertile eggs have been hard to come by until this brood so it would be a pity to waste them. Also it'll be easier if she hatches some than to try and break her broodiness any other way! I'll also give her some more ducklings when/if they hatch because with her (lack of) brain cells, I'm sure she won't notice a few more!
H