I honestly didn't think she was the mastermind either. My biggest hang-up was that how do you act that kind and supportive towards Yotsuha to her face while secretly holding that much hatred? You'd have to be seriously committed to keep up the "best friend" act for that long without letting it slip.
That said, the author has been quietly laying the groundwork. Fuu asking for the watch always felt a little suspicious. Rei's grandmother recognizing Fuu meant she'd been to his house before, which now makes a lot more sense if she was using him. The letter Rei delivered to Rikka comes back into play here, so that's another thread that was planted early. Then you've got Professor Futatsugi's weak heart, Fuu and Yotsuha sharing the same eye colour, and a bunch of other little details that seemed random at the time but are starting to connect.
So I don't think it's a Heavy Rain-style unreliable narrator as much as it is the story withholding her true intentions. Fuu played the long game so convincingly that even with all the hints, I still didn't want to believe it.
Thanks for the translation!
