I would keep trying to reduce stress on her, keep watching things like eating, drinking, pooping.
I'm sorry. They have fragile bodies.
I think you will probably have to just endure with it, until it plays out. You can try holding her when she can't get up but that might make her more stressed...
In regards to how long to give that nutrient to her...you are in theory looking at what she's missing, plus whatever it takes to grow her to adulthood, since she's still a duckling.
So maybe you can give it to her a bit past when she's well, based on a percentage of the amount needed to get to...
There are videos on Youtube of people treating this illness, and does seem to look just like the ones on ducks with niacin deficiency.
In some of those videos they also did some therapies with putting them in pools of water for weightlessness for a very limited amount of time. However, they...