Ducks usually cover their eggs up when they get off the nest to eat and drink so they stay pleanty warm. Some of mine will stay off the eggs for an hour. That shouldn't have anything to do with a bad hatch.
There's really no way for me to know what happened,
But I will tell you excessive turning especially during the last few days can cause them to be mal position and not be able to hatch so if you were candling day and night that's a possibility.
Mama duck knows best. It's good to have her separated and protected from anything bothering her, but It is also best to make sure they still have access to swimming water. Mama duck coming back to the nest wet helps with humidity for the eggs to hatch.
Just a few thoughts. Sorry they didn't make it, I know that sucks when your looking forward to little ones.
Ps. I am not an *expert*