(AAAAAAAHHH)
This is really good. It's quite surprising how grounded the story is in many of the real life aspects (not so much in the stuff between Hime and Mina). The only Age Gap I can stand is the type where the younger one pursues the older, so this is fine, although I think it shouldn't work out. There are too many bad layers to this relationship (large age gap, daughter of former crush etc.). I'm sure they could find different lovers, but it's not too serious right now anyway.
Honestly, those moments when Mina talked to Yae or loathes herself for seeing Hime as a replacement hit me pretty hard. The manga gives very good reasons for why this is a "test" more than anything else.