Lacrystol,
It can be confusing. I have read a number of posts, and there seems to be some variation in how ducklings react to different feeds. So I think that some are more sensitive to a lack of niacin, for example, than others.
I do understand that long term, too much calcium for ducklings or drakes is a problem on their internal organs. So it may not show up right away. Layer is not recommended for ducklings. In a pinch, it would be better than nothing.
Go with your intuition on the medicated chick starter question. Since I was not sure it was okay, and it was easy to find turkey/waterfowl starter crumbles, that's what I used. I also chose to follow Storey's Guide to Raising Ducks recommendation to reduce the protein from 27% to 20%. I used chopped up dry rolled oats one cup oats to 4 or 5 cups of crumbles (sprinkled a teaspoon of chick grit in to help digestion). By the way, a number of competent duck parents disagree with the philosophy of higher to lower protein percentages as the ducks grow.
A number of ducklings have nerve problems if they don't get enough niacin, which is three times the amount found in chick starter. Some duck stewards sprinkle brewer's yeast, a nutritional supplement, on ducklings' food. Some add 100 to 150 mg of niacin in capsule form to their water the first 8 or 10 weeks of their lives, especially if the ducklings begin to display signs of deficiency. Bugs and worms and such contain niacin, too.
It's a bit of an art, coming up with a feeding program that works for you and the ducklings. I hope that the various opinions, based on experience, will give you some ideas that will be helpful. As long as you observe the ducklings and make changes if they seem to have troubles, I feel that you will all be okay.