Why would she refuse vegetables, because she's a Cockatoo. That's what they do. Now if your especially crafty and patient you can change this, maybe.
The seed diet needs to change but until that happens she is at least eating. The only thing you can do is to continue to offer new things. It could take six months or more for some new food to interest her, then again, there is the not interested ever thing. We've had our M2 nearly 12 years and all green veggies are apparently viewed as poisonous to her as nothing green ever goes into that beak. Fruits are a different story, she accepts many most of the time. She loves cooked eggs and yogurt, again, most times but not always.
We feed Zupreme natural pellets. Others here use different brands. Originally we used the Fruit blend but the red poop stained anything it came into contact with so we stopped that. Plus it's hard to tell their health if the poop is red.
Know that whatever you feed her, probably half of it is going to go on the floor or bottom of the cage. That's just how they are.
As far as the seed diet, there is too much fat in seeds for our perch potatoes. Seed diets can cause fatty liver disease and death. We've had a couple birds die from this because their previous owners only feed seed diets.
Finally, time to a Cockatoo moves at a snails pace. Your 3 weeks maybe be an hour to them.
Forgot your last question. Yes you are. LOL