I still think her first revival was also to the time paradox, which was created after ciel/roa died, and thus having Roa already incarnated in SHIKI. But why the paradox existed in the first place has to have something todo with Ciel's strenght, or maybe Roa's desire to keep possessing her. roa was quit fond of her body, so maybe that has something to do with it. In the end Ciel is part of Roa's existence, not just Roa's life, since Ciel had to kill Roa's axistence rather then his life, meaning that Ciel became somehow part of Roa's imortal soul?
It might just nog be explained, since it is just a story in a certain setting where such a world is the truth, so there is no need to have a good explanation for everything, then again Nasu usually explains a lot of his worlds as the philosophy behind it is an important factor for the story. But in the end it would probably create contradictions if too much was explained in detail.
You are here alone again
In your sweet insanity
All too calm, you hide yourself from reality
Do you call it solitude? Do you call it liberty?
When all the world turns away to leave you lonely