Another source for help may be a waterfowl rehabilitator - they might have resources and information for the specific injury. You can get oral antibiotics at a feed store, should you decide to try that route. I would not be hasty to euthanize the duckling at this point - I would explore getting help with antibiotics.
Does anyone think that lancing the swollen place would be worth considering?
Does anyone think that lancing the swollen place would be worth considering?