I followed Holderread's model and gave mine chopped rolled oats from the beginning, so no problems with them eating the oats. I also supplemented with vitamin mix.
I think you are on the right track - give them some extra niacin, what you have, and look for the plain niacin capsules. I guess I was blessed to find plain niacin at our local drugstore.
22.5 mg per gallon of water is about a quarter of what they may need. But we are making some estimates...
Welcome to the Duck Forum!
You are among the not-so-easily-intimidated, so don't worry about questions or lengthy posts.
You have not (yet ) doomed your ducklings. And you're right. You want to give them the best start.
So.
I would get a bag of unmedicated chick starter if you cannot...