It probably all is related to what you are feeding. I would get them off of that feed as soon as you can (30% protein is just too high for ducklings).
If you have some of the regular Purina chick starter, switch them to that and also add a source of Niacin like Brewer's yeast sprinkled on the food. If your feed store has Purina Flock Raiser, I would put them on that instead of the chick starter. It is 20% protein and has added Niacin.
When I mentioned supplementing the feed, I meant with oats and chopped greens. You can mix up to about 25% oats (rolled or whole) into your feed. I also give mine chopped greens daily, usually leaf lettuce.
The foot problem could be the excess protein and/or deficiency of something else like Riboflavin (which yeast also helps). The other ducklings dying I am sure is nutritional as well. Dave Holderread in one of his books describes almost the exact scenario you are describing as a B6 deficiency.
I would change the feed as soon as possible and then go from there. I don't know what to tell you on splinting the duckling with the foot issue. I have never done that, but it could work. Good luck to you!