Could be some soft of genetic abnormality, but that "bowed inward-leg" looks similar to how a niacin deficiency duckling would look. You may read, and do this treatment in this thread.
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/
That description sounds exactly like Gimpy! I started him on niacin this morning. I dissolved a niacin pill in some water and fed him dipped lettuce. I didn't notice any dislike of the taste. In fact, all 4 ducklings fought over the dipped leaves. I think Gimpy ate niacin dipped lettuce until he was stuffed. Being 2 weeks old already, I expect it may take another 2 weeks to see if there is an improvement.