He's reincarnating as villains? I thought your theory was set on him being immortal and taking on the identities of the major villains from each other games.

You'll need to refresh my memory on how XII is the affect of a time paradox. The only games I know of dealing with time as a major plot point is VIII and XIII-2, the latter actually being about time paradoxes and VIII simply being about compressing all time.

In IX Garland isn't even the big bad, it's a toss up between Necron and Kuja. From a planning stand point Kuja is the big bad, but from a final boss stand point Necron is, convinced that Kuja's seeking destruction was the correct answer and planned to carry out that, but he's got zero back story or foreshadowing and largely disliked by the fandom in general because he doesn't make any sense being in the game.

VII gets a little convoluted at times. Sephiroth is running the show, but everyone with Jenova cells have uncontrollable urges to rejoin Jenova. He ends up using Jenova against the player as a boss to forestall them at one point. But Sephiroth is the prime figure and besides going a little insane learning the truth about the experiments, he was in charge of things even if he was just trying to be just like Jenova.