I'd prefer the idea without the demon, because it's a nice idea without further complexity and crossover but I guess it's a matter of personal taste. For me the idea of a -dormant, more on that later- split personality has more depth than the possession-idea. You could still focus on the crown - maybe Singer thinks he is the original owner of the crown but actually isn't. So she could try to follow in the footsteps of the crown-bearer, you could develop its story/background without actually needing an ancient being to be in your Story.
I'm not sure if that's helpful anymore, but as I was...