i am also surprised by this developement. I hope that the story will not dragged more than expected like Juujika. It's already good that we not learn that Okaya is the near invincible leader of a depravade cult
With the recent release of volume 4, we approach to the serie end if author kept the planned 5 volumes story. I had thought that before confronting Okaya, Maria will take down Ajiki before. Seems that Okaya will be stopped and punished more early than planned (unless that author repeat the first Kyou-Uruma confrontation and drag more longer the series by a timeskip and more useless chapters for some reasons, probably for money and editor wish XD)
I imagine a unlikely scenario if 5 volumes story still happen:
Maria succeed to takes revenge on Okaya by killing him, reveal his parents's crimes for assist their son horrific action and restore Taichirou's reputation. Maria is still on the run after having revealed as the culprit behind the four murders.
In the last chapter/epilogue, we see Ajiki in a another city, under a new identity, trying to led a new life hoping that Maria doens't found him also for revenge. Unfortunaly for him, the latter lead him to a fatal trap and is killed in result, achieving longtime goal for revenge of Maria/Marie.
For me, if the author kept her original plan, it could show how Maria will deal with Ajiki for finally complete her revenge.