I think it was more along the lines of “Hey, one of the people on this bus just so happens to be the smartest person in school! Maybe she would help us keep control of everyone.”
All of the heroines say farewell to Riki in some way or another at the end of their route, so they probably realize the secret of the world after their regrets are gone. Each character has almost complete control of the world whenever Riki focuses on them; notable examples include the existence of Midori, and the rebirth of Kudryavka’s family. Apart from Yuiko, Kengo, and Kyousuke (and maybe Komari) none of the characters show they are aware of their ability to manipulate the world. The reason they have this ability is that they are the people who are dreaming the fake world~
The most obvious example of Yuiko having control in other people’s routes, is that she is able to teleport in every route~
