Melinda, excellent analysis. I agree with you: I see Mia as the story’s lead protagonist and Sebastian as the Attractor. Mia goes through the biggest psychological transformation, the story is told primarily through her perspective, it’s her storyline which provides the core of the Plotline.
That said, it’s easy to look at Sebastian as a kind of dual protagonist. Indeed, I would argue it’s important to consider what he learns and what his arc is, if any.
Keith as Trickster: Yes!