Good job caring for this injury. Poor baby.
We have a pond that is full of snappers. Nasty things. If you can't 'trap' them can you legally fish for them?
Frankly, I would give my dept of conservation a call and tell them what happened and ask if there is anything they can do to trap and relocate the turtles because if they keep attacking your duck you are going to be enjoying a LOT of fried turtle and turtle soup.
Many states have 'nuisance' laws about wildlife that attacks domesticated livestock or damages domestic property. I'd check into that because the problem is just going to keep going on and repeating unless you are able to rid your pond of snapping turtles or move your ducks to a new turtle free swimming hole.
Good luck.