I'd stick with poultry feed. Drop feeding oats because you are diluting the amount of protein ducklings need to grow properly. If you go with Nutritional yeast in tablets just crush up enough to make 1 tablespoon and sprinkle over their feed. 1Tab to each cup of feed . I use chicken feed and sprinkle the Nutritional yeast over top of the feed.
Bless you and your moms hearts for saving these ducklings They are sure a blessing. And spending time with them maybe just what they are needing to get them back to eating their feed again, Enjoy them and post pictures when you can.