What makes cedar so aromatic is attacking their lungs and therefore weakens them. Get pine shavings instead. Just because two ducklings are doing fine does not mean they are not niacin deficient. I don't think they all have actual leg injuries. That is just against the odds. Ducklings need niacin supplement in their drinking water for their first 10 days of their life, after that weekly until 3 months, and after that monthly. Don't rely on the little bit that is in their feed. If it is niacin then they will improve fast. The only issue is that they have been having issues for so long now and you wonder how much will be permanent. They are still young so I think you still have a chance of fixing it. Don't wait. I would also add vitamin A. A good way of giving them the essential vitamins for now is to get a poultry supplement or give them Polyvisol without Iron for infants drops. One drop into the mouth 3 times daily for two weeks. Plus the niacin, which I would mix 200mg per gallon at this point also for 2 weeks. 100mg per gallon is normal dose, but they do have issues so I would double it. Don't worry this will not overdose them, since it is a water soluble vitamin. To avoid more raw spots try to get something you would use to cushion your feet with to avoid calluses from the pharmacy. Switch your feed on the suspicion they is may have mold spores in it. Mold will also cause leg issues. Do not feet them medicated fed at this point, if you are doing it. I think this is the best advise I can give you right now. It does not hurt to apply it and see, because what you have been doing so far does not seem to work so you have to take more action.