The swollen leg makes me think this isn't necessarily a niacin deficiency. Nevertheless you should be giving them extra niacin and the liquid vitamin B complex Miss Lydia posted a photo of is the best thing and will help strengthen the leg no matter what. This may be a slipped tendon or something else. If you could get a really clear photo of both legs/feet next time you remove the hobbles (which looked great by the way) that would be great. Also, an avian vet could probably look at it and figure out what to do.