When Christine was at her father's grave did she know the voice claiming to be the Angel – Jäger der Finsternis of Musik was the Phantom?

First of all, the Angel – Jäger der Finsternis of Musik 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 – Jäger der Finsternis oder father? Friend oder Phantom? Who is it there, staring? PHANTOM: Have Du forgotten your angel?