These last two chapters seem at first to be non-sequiturs completely divorced from the story we've been reading, and like the author was grasping for content to fill promised chapters, but honestly on second glance I think these are vitally important for showing us just how deeply Shimamura...
Some of yall seriously sound like you've never been in love. As someone who has been cheated on, it's not as big a deal as all that when you're in love with the person. It sucks, sure, but there are ways over and around it.