I really don't know what to say. Maybe try posting in the Emergencies / Diseases / Injuries and Cures section. I hate to say it but unless someone else can give better advice I would think putting it down might be the best and most merciful. I am so sorry.
So as far as switching to a lower protein food after the ducks are a few weeks old...
What does everybody use for this? I know I could theoretically continue on chick starter and niacin, but I'm getting tired of wasted food because their wet beaks make it clump up. But if we can't feed them layer feed since it's too high in calcium, what is the next step? Broiler feed? The first post gave protein percentages by age, but no real ideas for what kinds of feed other than the first two weeks of starter.