You know... generally I would feel like the presence of a mastermind at such a juncture would be... "forced".
But when I look at all the events that have happened, how certain things should not be possible (like Mayoiga knowing about such a class of rituals), certain... "coincidences" like a girl with a deep void and absurd potential being born, losing her parents, having her baby possessed by the vengeful spirit of the slain goddess... things click now.
I will point out... Kanata being at the right place to be possessed may have been truly a coincidence, karma working its will... I forgot now if that is the reason why Kazuya is a "taboo child", but it certainly helped.
One way or another... it is clear now... they are dealing with a master manipulator whose timetable moves at centuries at a time.
Thinking better... this reveal actually makes the whole manga much better, it means that many events that lead to this moment were carefully crafted by Ayahime.
And I will say it now... chances are she has some power to see into the future, but MUCH stronger than Arumi.
I want those curse allies to be killed... not like they are doing anything Mimane hasn't done... would say Mimane did worse even... much worse,,, the pervert geezer too... and not like Arumi is a dainty little flower either... hmmm... what was my point? W-Well, anyway, I want them dead. (god, the sexual deviancy of this manga is burning my brain)