Aside from that, I suddenly realised that instead of the wooden Tsumugu, how is the situation viewed from the opposite perspective? If what you say is correct and Tsumugu is so obviously in love with Chisaki, how much of it does Chisaki see? She's always so unsure of herself and doesn't think much of her own worth that does she feel guilty of having lived in the Kihara house for so long yet not responding to Tsumugu's feelings? Obviously that sense of paying back must be a part of why she's so dutiful, taking care of the geezer and preparing the bed for Tsumugu's boss, but is she feeling she should go beyond that? You could certainly interpret her behavior and expression as guilt when she was getting back home and met Tsugumu at the door. As if she was betraying Tsumugu by meeting Hikari (even though Tsumugu encouraged their meeting).