The story really didn't need the Celestus. Embracing pre-Avacynian traditions/magic with Sigarda leading the rituals was sufficient IMO.
I love the theme and vibe of this set as an organic Halloween/nature horror world lead by Sigarda embracing more occult forms of power that are traditional for a world like Innistrad. Can't believe I'm saying this but I feel like this is the best Innistrad has ever been, close to the roots of the theme. The frost, pumpkin patches, coven of witches, candlelit nature rituals, I'm really feeling it.
My friends and I have a theory that Emrakul is the only true Eldrazi, and reforms Ulamog and Kozilek on every plane she visits. Brusella began to resemble Ulamog, and with enough time, may have become him for all we know. Or a new version of him. As the creator Eldrazi, and only undefeated one, it may make sense that only she needs to survive.