Sarah Gadon IS in Amazing Spider-Man 2 briefly. She is NOT playing Mary Jane Watson, who will NOT appear until Part THREE.
(Yes, it's the same link from post #142, but apparently it's just been updated.)
Remember that Marion Cotillard lied and said she wasn't playing Talia in TDKR in her interviews.
SONY might have told her to deny it because of some of the negative reactions. We won't know for certain until the movie is released.