When Derek says that he hides no secrets from Odette, they zoom out and Odette's shirt changes color from blue to pink.
In this movie, Odette's dress is blue. It changes to the white and green one from the first film, during the scene with Derek when they are doing the 'Festival Days'.
Whizzer, according to Zelda, can imitate anyone's voice after hearing it only once. Yet, he tricks her by mimicking Rothbart. As Rothbart died in the first movie and Whizzer is absolutely ignorant of Zelda's intentions and the existence of Rothbart and the Dark Arts until she tells him, we can conclude he never met him, much less heard his voice. How could he imitate Rothbart?
Zelda says the notes are needed to access the Forbidden Arts. Yet, in the previous movie, Clavius got them without the notes.