Another thing I don't quite understand yet is... weren't the Ascians in the process of ruining their own plan?
The way the Exarch describes his future, what happened when black rose was used wasn't a calamity, but a world ender. It seems like life on the Source was on its last legs with no hope in sight. This means that, had the Exarch not changed the future, life on the Source would have ended, which would have torpedoed the Ascians' plan. With no people around, there would have been no way to trigger the rest of the needed calamities. And even had they somehow managed to cause the rejoining anyways, there would have been no more life left on the Source to feed to Zodiark.

And on that note, is it even possible for the Ascians to succeed any more? Even if they manage to rejoin 12 of the shards, that still leaves the void shard. Emet himself admits that they screwed that world up something fierce, so doesn't that mean that, even in if everything else went smoothly for them, they'd be forever stuck at 13 out of 14?