You might be onto something. She was "Takagaki" (according to Shizuki) in middle school, but in high school she's "Hebikawa". Plus, I remember her mentioning to Kusunoki that she doesn't like being called Hebikawa - implying that she may not like her stepfather.
I feel like the author's intention is to give off the feeling that the mum might return... but in truth, she never will, and it's just wholesome fluff all the way.
Those are dramas from the start though, aren't they?
Here we have a series that started as a wholesome childhood friend romcom, that suddenly, without any warning, took a 180 degree turn and added unnecessary drama.
By this point, the only way this nuclear dumpster fire can be redeemed is if, at the end of the arc, back in the present, Yuu pushes Ayami away, tells her to fuck off, and loudly proclaims his love for Hikari.
It's down to 7.60 now. 189 1-star ratings.
Crazy how one bad chapter can cause a manga's reception to plunge.
It's well 'earned', IMO. Story was going so well right until ch. 21 flushed it down the drain.