Some questions from my NG+ playthrough

[Anonymoose]

First of all, what is Sabik?

The best clues we have are that Sabik is a chunk of "black auracite" somehow connected to Zodiark's power, which can manifest as the ultimate destructive magic, "Ultima." We still aren't completely sure about its true nature.

If we could have rescued Minfilia from The Antitower, why didn't we try that with Y'shtola in the first place? Is it because The Antitower was unknown to us at the time?

If I understand the question properly, the missing Scions were thrown in different directions and could have ended up anywhere. Minfilia's destination was deliberate; she jumped in with the intent of being drawn to Hydaelyn. But yes, we were mostly working with what information we had at a given time.

Third, what was it that undid the Allagans?

You can more or less trust the story you were told in Crystal Tower (at least, for today). The stagnation of Allag provided the opportunity for the Ascians to move towards Calamity. The mess they stirred up included the resurrection of Xande, his plan to fulfill his lost ambition of conquering Meracydia, his pact with the void when it was going poorly, the primals summoned to resist Allag (including the Warring Triad), the defeat of Bahamut, the resurrection of Bahamut as a primal, the capture of Bahamut to create Dalamud, the conquering of Meracydia with that supplementary power and the void, etc.

Xande became nihilistic after the fall of Meracydia and conspired to allow the Cloud of Darkness to consume the world. The Warriors of Light of that era laid siege to Mor Dhona and took the Crystal Tower itself. Cornered at the top of the tower, Xande discharged all of Dalamud's energy into the tower at once, but the tower buckled and discharged all its energy into the land, triggering a series of massive earthquakes that devastated much of the world.

An empire is a massive network of resources. The earthquake ruined enough of the networks and supply lines that the rest collapsed. There was no way to maintain its cohesion. (Could some starships have gotten away...? Mmmaybe.) But look at how much damage a global pandemic did to our supply lines. H*ck, remember when that ship got stuck in the Suez? Even that mucked things up a bit, lol.