irenic influence, Innilis willingly became pregnant by [Gratillonius]."
-Poul and Karen Anderson, Gallicenae (London, 1988), p. 18.
Looking through the King of Ys tetralogy for information about the Council of Suffetes, I meanwhile find other information, e.g., I did not recognize the word "irenic" and do not even remember noticing it during previous readings of the series.
It seems that the Ysan Gods allowed first Colconor, then Gratillonius, to become King because they were preparing to end their covenant with the city and to withdraw before the new God of the Piscean Age. Even if the Three did not control who became King, they certainly did choose which of the vestals was to be marked as the new Queen when one of the Nine died. A Christian could not have become King because Christians are monogamous. However, neither a Christian nor a Mithraist could possibly find it acceptable when his own daughter was marked. (Indeed, although I believe the incest taboo to be a social construct, not a divine prohibition, I am happy to remain bound by it.) The rejected Dahut became the instrument for the destruction of the city. But the Gods broke faith with an honest King when they marked her in the first place. Christians would say that such behavior is typical of pagan Gods whereas pagans retort that the Christian God is intolerant of ancient freedoms.