After watching the video, I noticed that your duck walks like a chicken I took care of. I could feel nothing wrong with her legs, but she just couldn't find her balance. she would lean backwards and fall if she went too fast. I thought it might have been Mareks or something. I never found out what it was, though. All I know was that isolating her, putting her in a sling for a while and taking her out to let her try to stand and walk on her own for a while almost healed her completely. Once she could wobble around a bit, I took her outside to range with my other birds and that worked wonders. Never underestimate the magic of fresh air.
The only reason that chicken is not alive right now is that a fisher cat killed her when she was super close to recovering.

