Hi Agregg,
How old is your baby now? When did it hatch, and was it assisted? (Sorry, I'm on severall threads, and I get lost on what I've read where)
Something needs to be done quickly for that duckling, Hopefully someone can help you figure out whats best for the leg. and so sorry about the duckling that died.
this is for chicks but maybe something there that will help your duckling. https://sites.google.com/a/poultrypedia.com/poultrypedia/poultry-podiatry