I just candled them all again and removed 3 of the 6 that were definitely going nowhere. Post 42 #1 and #2 and Post 43 #1 are still the nest, but I don't see any signs of movement or hear anything. I'm going to give them another 24 hours and then clear her out. I feel sad about the whole thing, much sadder than I would have thought I would feel.
I'm going to monitor their diet better and let her have another chance (if she wants one). We've (accidentally) added a new male to our little flock, and this duck she sits on the eggs of her own and another female, even if one of them did have a genetic problem--which I don't--I think she would still get a few ducklings out of it because of all of the different combinations. I suspect her diet may not have been perfect and she was just a first-timer. She's been in the nest for 45 days now, so I'm guessing she wasn't doing a perfect job of regulating temperature/turning, etc. Next time, I will also make sure that none of the other ducks or geese can get near the nest in case they disturbed her. Anything else I should consider?