Why do you want to feed just corn? Cheaper? Healthier?
If the adult ducks really do not eat much, just offer them the same feed the ducklings get--it will make your life easier to deal with only one kind of feed, and will be fine for the adult ducks.
If you want to save money, skip the mealworms, and just offer duck food. (Unless you're raising your own mealworms; but they are quite expensive to buy, if you look at the per-pound cost.)
If you want the ducks to be healthier, I also recommend duck food.
If you're treating it as an experiment and just want to know what happens--go ahead. You've got several people already predicting the results, and you can find out if they're right. Or you could do a different experiment by offering duck food AND corn, and see if they're clever enough to eat appropriate amounts of each.
About whole vs. cracked corn, as long as they have enough grit in their gizzards they should be fine either way. Cracked corn supposedly loses more nutrients in storage, so I'd use whole corn. (Except that I recommend duck feed.)