Dan Hiri Weller was an advocate of the rights of children of interracial (human/Mazoku) marriage, and he founded the Nameless City in Shin Makoku where they all lived as refugees. (I only vaguely knew what he was up to before, but episode 47 elucidated his past.) Of course, he has a special interest in this because of his son, but it seemed that he was also driven by the need to make his mark in the world. So, this could be another reason why the Shinou wanted Cecilie to be Maou. Their marriage was one of many steps along the road to peace between humans and Mazoku. (Many, many steps…)
So it could be that peace is what the Shinou wants after all. Maybe the Shinou, like Yuuri, is an idealist, and that’s why the Great Sage is there for both of them, to temper that idealism with reality. And maybe the Shinou was just trying to help Yuuri to be realistic, and so Murata had to go the other way in order to bring everything back to balance.
It’s all speculation, really…there’s kind of an eerie, almost evil aura in the Shinou’s temple, but other than that, and Murata’s offhanded comments (like, “He changes his mind a lot”), we don’t really know anything about him.