What your probably seeing in your duck is a deficiency in niacin. Not sure if a hobble will work but maybe worth a try.You are on the right track. I gave vitamins and kept giving mine the 'starter pellets' sprinkled on diced cucumbers, shredded lettuce, etc. She is up and walking with a limp but strong! She sometimes puts the left foot over the right foot so I am trying to make or purchase a duck hobble to keep the legs apart. Found out that some of the starter feed does not have the correct nutrients and vitamins. And this causes the problems with walking for a lot of ducks! Anybody: How do a make a duck hobble???
Here is info on niacin deficiency http://metzerfarms.blogspot.com/2018/01/niacin-
what-is-it-and-why-do-my.html
https://sites.google.com/a/poultrypedia.com/poultrypedia/poultry-podiatry