I came across your article while preparing to do my own. I recently had a young guinea keet come down with wry neck. I asked a friend whom works at the feed store I use and she told me to try poultry drench. I picked up a bottle at tractor supply and used a dropper to give the keet 2 small doses...
How about wry neck. Not sure if ducks can get it but sounds like some of the symptoms. Heres a link to an article:
https://www.raising-happy-chickens.com/wry-neck.html