That's close to the age my runners (descended from mallards) started with the greenish highlights. Zehn's bill changed color, quickly it seemed to me, when she was about a year old. So the color change was pretty quick (unless I missed something).
I wonder if the trauma of the attack may have affected the duck's biochemistry and caused the bill to change color, or if it was going to change anyway and because you are watching so closely you noticed.
Regarding the eating - after trauma, a body sometimes does not provide much blood flow around the stomach, so not eating is not too worrisome unless it goes too long. And too long is something I just use my instincts to decide. So, I'd offer just a little of a favorite treat to see what happens, but mostly be sure she's getting plenty of clean water, in a safe clean (I know with ducks it's relative) environment, and vitamins/electrolytes/probiotics in addition to careful wound care.
Goodness. I pray all will be well soon.
Poor ducky.