Is there a time limit on treating spraddle leg?
And what quality of life will the duck have if it is not fixed?
It should be fixed as soon as possible. The younger, the better. How old is your duck with the issue? Their legs are very pliable in the first week of life, but after that it gets harder to fix such issues. If not fixed, the duck might not be able to get around as an adult and quality of life will really suffer. Can you post pics of your duck's legs?
I would give a hobble a try and continue with added niacin. And it is not traditional yeast, it is brewer's yeast or nutritional yeast. The autocorrect may have been working, but baking yeast is not good for them, it's not what helps, it can harm them.
A hobble is a very flexible cord with loops at each end. The loops go around the legs of the duckling, and the cord is short enough to keep the legs from splitting too far apart.
Considerations are - the hobble needs to stay on, not cut the circulation, and be the appropriate length for proper leg width.
If you have Storey's Guide to Raising Ducks, there is an illustration. Or you can look at poultrypedia. https://sites.google.com/a/poultrypedia.com/poultrypedia/poultry-podiatry