When Christine was at her father's grave did she know the voice claiming to be the angel of music was the Phantom?

First of all, the Angel of Music is a character in the story her father tells her, and he promises her that when he dies he will send him to her. She believes the Phantom because she LOVES her father. It has nothing to do with how smart she is.

PHANTOM: Wandering child, so lost, so helpless Yearning for my guidance CHRISTINE: Angel or father? Friend or Phantom? Who is it there, staring? PHANTOM: Have you forgotten your angel?