In addition to antibiotics, and until you can get that, I would put a quarter teaspoon dried oregano per cup of moistened food. It is a natural anti pathogen, used by some larger commercial farms.
And turmeric is a natural anti inflammatory. Safe for ducks. Half a teaspoon per cup of food...
No, I don't think she will be safe. Drakes will team up on a female. Glad you are caring for her. Sounds like you will need to manage them carefully. You can introduce them, with constant supervision, and intervene if the drakes give her a hard time, keeping them separate when you are not...
The infection may have gotten into the joint, if that's what it is.
What I would do:
If I could get him to a vet, I would. Meanwhile,
Twice or three times a day if possible, give him time to float in lukewarm water for 20 minutes or so - that will take weight off the leg and improve...