A very good read, even though I've seen only 4 of the 8 endings so far, I may need a break before I go for the other ones.
Anyway, I find the "answer by showing your emotion" mechanic to be quite fun and interesting, it's like you play as one of these silent protags! (Wait, Kokoro IS one in here... uh)
I think you've nailed every aspect of a VN (hit the nail's head perfectly flat into the board even, ): the character portraits and the songs are indeed very cool, and the writing is solid as titanium.
So looks like Kokoro being silent makes it easier for others to talk and open up to her, I don't know why but having her be a listener is funny to me.
Though, I wonder why Chapter 2 lasted way longer than the others, it took half the time of my one and only playthrough.
But yes, very good stuff overall!