So, meteors fly through the Skaian defense portals leading to the home planet of the players, right? And both exiles and paradox clones use these to get to the home planet, right? And the time period the portals lead to varies, so some paradox clones become ancestors while some end up like A2 Damara, arriving after the apocolypse. So what's stopping the same from happening to exiles? Isn't it possible for an exile to arrive before sburb starts, maybe even changing history or a player's entry?

I would be technically feasible for an Exile to do that, or even a Player. The way Homestuck’s Timeline works, though, this wouldn’t ‘change’ the Timeline, it’d have been the way it went down in the first place, though. Also the deviation seems to be rare, Damara, Roxy and Dirk being sent so far ahead isn’t quite the standard.