I have used that waterer for ducklings and it works pretty well for about a week. I modified mine with a container on top to auto fill. I tend to make larger waterers as they grow. As they grow fast, they tend to outgrow their waterer. A modified Tupperware container is one of the best ways to go as they get older. With the size yours is, it should still be fine with that waterer, but you will want to plan for when it out grows out.
His brooder area could be warmer than the room temperature. I would shift the heat lamp to one side, rather than in the center. This allows them to go from one side to the other to cool down if needed.
Ducks sometimes will also sneezes when eating. If they accidentally inhaled some powder, they will sneeze. Especially with crumble, it can be a bit more powdery. They overall usually sneeze to get something out of their nares, or when cleaning them.
Ducklings get easily tuckered out. So as long as your cutie is eating and drinking just fine, your duckling is likely ok.