i'm guessing it's not gonna end up being mao but ruirui instead
I'm thinking it might be Futami-san herself, manipulating her authors to keep them isolated and at constant odds with each other, thus making them vulnerable and leaving her the only person they can latch onto for support.
This was also my first thought. Her whole game is giving them just enough attention to make them feel special and loved and then using that to turn them against each other and make them feel like they have to outperform everyone else to prove they're the most worthy. What Futami said doesn't necessarily exclude herself.
However, the username does kind of point to ruirui and that "working group" was 100% about distracting and scoping out Nanakusa, so it could also very much be one or both of them.