well, I think the author had other plans at the start (with the sister, for example), but the nearly two years hiatus probably led them to end this quickly and rush out a conclusion. Haruka suddenly discovering love out of the blue, because Sensei hid her shoes is a bit forced.
I guess the author lost interest in drawing manga and only finishes it to fulfill contractual obligations.
Agreed, Haruka's poor behavior probably had something to do with her living conditions and she might have changed if her things with the younger sister was explored.
I guess kudos to the author for at least giving us a closure, there's so many other manga out there that were just left in hiatus forever