Hebikawa doesn't actually like Shizuki, though. She doesn't dislike him, but her ultimate objective is to make Kusunoki jealous and expose her darkness.
That is a fair point... but is he so insane as to invent memories of childhood friendship with Nagare? Since again, pretty sure they only met in high school.
I'd have to go back and reread the start, but I'm pretty sure he originally started getting bullied to save another kid, and was collecting evidence until he could expose them. So this "twist" directly contradicts the start of the story.
Nah there was a flashback early on, chapter 4 or 5, which showed a young Rei and Shidou at a fancy dinner, sneaking food under a table to eat. Shidou has forgotten this, but now he might remember.