Responses
Yup funny thing is Flowerchild's previous works did touch upon such theme's read "Eve's Medicine" and "Endou Yasuko stalks the night".
Let me also add this the fact that they are back together immidiatly after an impulsive breakup with none of the problems fixed is just another hint that their relationship inevitably goes downhill. So yeah it already backfires on them.
I bet in the end sei and aya will just split apart like nothing happened. This impulsive decisions seems romantic but surely it will back fire to them both... Proven and tested.