I actually ended up catching onto it being her around when the investigation started. But then again, that could’ve just been me recognizing unreliable narration when I see it. There were a lot of points while they were setting up those cameras where you could tell something was up. By the time she’s giving Shuichi that speech in the elevator, my suspicions were pretty much confirmed.
In any case, switching to Shuichi at points before the reveal would’ve given it away too soon and I feel like it wouldn’t have really worked for most people. Even if one had trouble seeing through the entire trick because of the unreliable narration, I found the twist to be fun. And honestly, that’s something I personally appreciate far more than complete fair play.