Very sorry for your loss. I have never had a duck die, it is so sad when ever I hear about ducks dying. You should not feel like you are letting your flock down. You did everything right, and sometimes unfortunately things happen. :hugs
I try to mix their treats, with their normal feed. I would cut them into small pieces, I mean it cant hurt. I give the treats in all 3 of their feedings.
I got an egg incubator, and I want to start hatching, which type of duck do you thing I should Hatch. These ducks will be separate:) from the older Blue Swedish ducks.