Sorry, I was unaware I was tagged twice here.
Could we start off with a video of the duck walking/ or trying to walk? In case you're unaware how to do that, you'll need to upload the video to a video platform like Youtube or Vimeo first, then copy/paste the link over here.
I'm not totally suggesting this isn't a niacin deficiency, but I want to set in on the back burner for now because the symptoms the duck is portraying, and its age doesn't really make me believe that might be the sole cause of her problem.
There are a variety of reasons that a duck may limp, to name several, being nutritional deficient in the vitamins D or B3, an injury, Bumblefoot, or an infection, which is something I'd like shed some light on here. I notice you say she had some respiratory problems earlier on, and that subsided with the use of antibiotics. In case your vet hasn't told you, there are several respiratory infections, for instance,
mycoplamsa synoviae, that can not be
treated only alleviated with the use of antibiotics, this is directly due to the fact that mycoplasma cells lack a cell way and are naturally resistant to most antibiotics.
By no means, am I suggesting she has MS, but I will say it does coincide with her other problems, and the infection is currently still active and is affecting the mobility in her hock. Which is a clinical sign of MS.
Here is a link on MS, and if you feel like that may be her problem, you should consider sending samples to either your state veterinary diagnostic lab, or try contacting a national diagnostic lab like Zoologix, for a possible diagnosis.
https://www.merckvetmanual.com/poultry/mycoplasmosis/mycoplasma-synoviae-infection-in-poultry
http://www.zoologix.com/avian/Datasheets/PoultryRespiratoryPanel.htm
https://www.metzerfarms.com/PoultryLabs.cfm
As far as some of the basic care you can implement as of now, that might consist of keeping her In a soft padded pen area, where feed and water is near, administering vitamins often, as well as letting the duck relive the weight of its legs via water therapy.
In case you haven’t already, could you gently pick the duck up and carefully inspect both legs for signs of swollenness, redness, warmness, bruising, or signs of bumblefoot on the foot pads?