Well, it sounds like you're doing great so far! Try to keep the duck resting, get some rescue/health supplements into the little quacker and I'm sure there are people on here who can walk you through splinting/wrapping!
Hang in there! :hugs
I found this:
https://healththerapy.org/snapping-turtle-bite/
Sounds like the swelling might be infection related. If that's the case a course of antibiotics might be necessary. I would try a poultry/foul rescue supplement first and see how the next 24 hours go.