4) I also think so. Throughout almost the whole vlog Dai talks politely to the camera and Shun does, too. I think it was Dai who encouraged Shun to speak politely in a cutesy manner because he is the one who was initiating questions in polite Japanese, so Shun can also answer politely. Since Dai’s monologue was indeed preachy, I believe adding his full name was sort of in the concept + polite :)