Try finding a rehabber to take her in. You are heroes, you are! At the same time, there is a migratory bird act law that restricts who can care for wild waterfowl.
Aside from that bureaucratic detail….
I will think out loud. I might consider oral antibiotics, and letting air get to the wound. Keep her away from flies, I think you already know that. What are you currently using to medicate her?
A good rinse in saline solution several times a day would be good, and an antibiotic cream, not ointment so that more air could get to the wound. I worry about anaerobic bacteria (gangrene) in those wounds (I am imagining a turtle took the foot).
Vetericyn is a good product for cleaning wounds also. Check to see if there are bits of sand or anything in the wound. Those can prevent healing.
There is a Duck Forum her on BYC - you may want to check in there, many knowledgeable duck people review that forum.
@casportpony