I kinda suspected that "random talented musician girl" in the flashback was going to be Subaru sensei.
There's much more to Subaru sensei than "horrible attitude perfectionist slave driver rival character". There were glimpses of it before, like when she heard the club's song and taught the sax girl.
She understands the suffering required for artists to flourish, but it made her cold and hard.
Honestly there's so much drama in just the school side of this story. I think the homicidal record producer plot really detracts from this mangas.