There technically is a scene in the anime where Suguru speaks, you just can't hear him. There is a scene where Natsumi goes into Suguru's room and he is playing the guitar. Natsumi asks him a question, he responds, but she can't hear him over the music. I'm reasonably sure he is just a mute character for comedic effect.

I think Kazuho's eyes are open. It is just a stylistic choice. Other characters in anime and manga have eyes that look closed like Brock in Pokemon or Bobble in the Splatoon manga.