I hope she does fine! Good luck.
I have no personal experience, but I wonder if you could find enough information if you did a search for medical information designed for areas without doctors--for instance, I know there are books, and probably websites too, that are dedicated to helping people in remote areas, third world countries, etc. find the information they need to treat injuries. Also, perhaps you can find resources for hikers and climbers--first aid for broken bones, etc. That kind of information could probably be applied here.
I agree that amputation is probably a bad idea without a doctor and anesthesia. But it depends on how much is left of skin and such. If it's only attached by a flap of skin, then it might be your best bet after all. It's really hard to say. I had a duck break a leg in half--we had to put her down because the injury was way too severe to treat--but if it had been a wing I think we would have just trimmed the flap of skin by which the limb was hanging on and then treated it with antibiotic and a wrap. It was literally hanging by a flap--no blood flow whatsoever in the separated part of the limb--so there would have been little danger in that.
Ducks are highly resilient, and I imagine she'll recover just fine if she's treated well and given time. You've gotten some really good advice already and I commend you on being willing to try. It takes guts and grit to tend to a wound like that with open eyes.
Good luck!