I get the impression that Shunsuke is the chessmaster archetype, and his sliminess comes from the fact that he has a plan that involves everyone, but no one knows their real purpose yet. I mean, if your goal is to go back in time to change the future, you have to be aware of the domino-effect of each one of your actions.
My guess is that Shunsuke has gone back several times now, and finally set up everything the way he wants it to be. In the process, he’s seen everything play out repeatedly, and knows how everyone will react. For example, he probably knew exactly where Yuu was going to be looking when he walked into Shun’s office, so Shun positioned himself to the side of the door just to mess with his little bro.