• The one choice that the evil Harry in Mirror, Mirror made differently back around Bianca’s party was being straightforward with Susan and inviting her as his plus one, rather than Michael.
I've heard mention of clues in the story referred to as JB hitting us with nerf bats. Personally, I feel like the clues regarding the choice at Bianca's party are more like JB hiring a team of professional baseball players to beat us into submission with real bats.
1. The opening chapter is Michael lecturing Harry that he needs to tell Susan he loves her or he is going to lose her.
2. Leah takes Susan's memories of Harry
3. Harry saves Susan when he tells her he loves her, which restores Susan's memory of him and allows her to resist the Hunger
The choice Mirror, Mirror Harry makes differently is to not tell Susan he loves her (#3). It's the central decision in the story from the first pages to the end.
