I don't agree overly much with you, especially Shinta. The dry ground people had to experience the beginning of a minor ice age, or something, in addition to losing all the contact with the people undersea, so it's not like they would have just happily and gleefully continued their old lives while the sea folks were hibernating. Objectively speaking the sleeping undersea people got the easy card out of the deal. So, treating Hikari all happily and not understanding his stress and suffering is nothing but a single side of a coin. Still, Hikari himself seemed to realise that after his outburst with Tsumugu.

The relationships really are a mess in this story. Chisaki certainly hasn't changed at all, aside from her age and body, and I reckon that's the reason why Tsumugu is keeping his distance most of the time, assuming he really is chasing after her like you seem to believe. I'm not so sure of that, but for the sake of the argument I'd reckon that he won't make bold moves until Chisaki changes also emotionally. Who knows if Chisaki can ever forget Hikari on her own or if pushing her would allow her to move forward or break her. I guess Tsumugu doesn't know either.