If it were my guess, I'd still say bumblefoot. Just in a different area? After soaking Sunnys feet in the iodine solution for a few days I already notice her bumblefoot healing. Instead of being swollen to the touch, the buldge is deflating. I haven't even started the neosporin yet either!
If the antibiotic treatment doesn't seem to do much, it could be a fungal infection, especially since you live in the south. Just keep doing what you are doing. If it is fungal, it won't respond much to antibiotics, and you will need to think about antifungal - I am not sure about what works best with ducks in that case. I always try to remember that what works for me may or may not work on another kind of critter