I think one thing to remember about this whole ordeal is the fact that they are in the dream world. Ergo, everything that doesn’t make sense is possible simply because they are in the dream world. This happens much more than in the Mio route. Kurugay and Kud are other examples.
It’s hard to argue exactly who chooses it to be so, but at least in the routes, I’m confident that the heroine of that route chooses it. So why does Midori just suddenly appear? Why does Riki just hear them in the middle of the ocean? Because Mio wishes it to be so. Because she believes that is how she can get Riki to help fix her regret.
