Luso also appears in FFT: War of the Lions.

Ok, I felt that only being a year old, FFTA2 info was still considered spoiler worthy.

Yes, I feel that's the best explanation for things at this point. The scions and the Lucavi are far too similar for it to be mere coincidence. In all likelihood, they are the same entity. Since they actually exist in FFT, they must have come from the otherworld to Ivalice at some point, and from that, the Rift is an obvious choice for their arrival. Seeing as we already see demons approaching from the rift (the Neuhkia), it makes sense that they could reach Ivalice through there as well, and given Lezaford's warning, the opening of the Rift could well have facilitated the Lucavi's travel to Ivalice as well as being the cause of the cataclysm.

How much of the change to Ivalice was the cataclysm, though, and how much was the war against the Lucavi? Mist seems gone from the world in FFT, though auracite remains. Was it all used up battling the Lucavi and whatever else came through, or was it wiped away by the opening of the Rift itself? And what happened to the other races?

I always forget that XII was in development for a year before FFTA's development started. They spent way too long on that game.