Also... I *know* I have seen that symbol before, and I think it refers to the Divinity of Mankind, but I can't recall specifically where.
It would make sense if it's the Divinity of Mankind. First off, there's a squared circle within the triangle, which bang-on as a symbol of the celebrants. Secondly, they have a triangle to represent their three different apexes: Experience, Intellect, and Intuition.
I'd also like to add that Isabella seems to have left us a clue (in case anyone else hasn't noticed). Note how on page 33, the alchemist fire is red. As well recall that the alchemist fire in the shop was colored red, as was the fire that burned the house.
Now, note on page 24, panel 5. The fire in our mystery lady's hand is without color.
Was that intentional Isabella?