I use nutritional yeast added to the crumbles of all ducklings: 1 tablespoon per cup of crumbles, as a preventative supplement. The cheapest source of nutritional yeast that I have found is Walmart own brand. I change to brewers yeast for laying females as I can buy it in bulk from Josh's Frogs on line. It would be incredibly expensive to supplement a flock of laying females with nutritional yeast.
If a duckling has developed symptoms like yours has, I treat with Durvet High Level Vit B Compound bought from Tractor Supply or other feed store, or from Amazon. The dose is 1ml per day orally for a 2 week old duckling. We recommend soaking it in a treat (mealworms) or a few crumbles and watching the duckling eat it. But it's difficult to be sure it's all eaten. I tend to give the full dose twice a day to be sure the duckling gets the 1ml dose.
I currently have 2 rescued jumbo pekin ducklings that collapsed with severe acute niacin deficiency on the second day I had them, despite getting nutritional yeast in their crumbles from when I got them. I gave them each 1ml Durvet soaked into mealworms twice a day for 3 or 4 days, but from then forwards added 5mls Durvet to half a gallon of water and used that in their water tub. They are each drinking half a gallon of water a day (they are 5 weeks old and huge, having doubled in size the first week I had them which was from their 2nd to 3rd week of life. As they drink so much I can be sure they have had their daily dose when the first half gallon is drunk, and they have plain water for the rest of the day.
I have no experience of using "pure niacin" although I have had to buy human Vit B mixture from Walmart pharmacy when Tractor Supply didn't have Durvet. The 1 ml of Durvet equivalent of Niacin, was in 50ml of the human mixture. Fortunately, I got Durvet from Amazon Prime the next day