"Seishun Butayarou wa Randoserugaaru no Yume wo Minai" (rascal does not dream of girl with backpack) by Kamoshida Hajime. This is volume 9 in the series, an account of how Sakuta is making progress in returning to normal family life.
After years parents and children living separately, this is the volume where Kaede's mother finally made effort to see Kaede again. The root cause for their separation was Kaede's amnesia, which had been resolved since volume 5, so this reunion has long been coming. What was not obvious was the ever growing distance between Sakuta and his mother, after years of Sakuta taking on what would have been his mother's responsibilities in taking care of Kaede. The first part of this volume details the happy reunion of the entire Azusagawa family, promptly followed by Sakuta's existence being made redundant.
This seems like a replay of volume 1, as told from Sakuta's perspective. Honestly I was hoping that this anomaly would be resolved with something that involves Sakuta's cat since that cat appears to be able to observe Sakuta at all times, but the actual solution was something different, and something fitting for a romantic light novel series.
The middle part of the volume was an alternative universe where every one was living a better and happier life, but nonetheless Sakuta chose the current universe. The purpose of small Mai (girl with backpack) appears to be to guide Sakuta between the two worlds, but it's still not obvious why that was important enough to earn her position on this volume's cover.
We will probably see more of small Mai in the future, as well as the new character introduced from the other happier alternative universe.
Previous (2020-08-03): Rambutan
Next (2020-09-22): "Seishun Butayarou wa Mayoeru Shingaa no Yume wo Minai" by Kamoshida Hajime