Decent into Avernus IS finished before the start of this game, even if it's not a direct prequel. In game you learn that the tieflings are refugees from elturel, that the other residents drove them out after the city returned calling them devils.
That sums up everything.
No, as discussed before: we know that the game says that tieflings are refugees and were driven out by others after the city has returned from Avernus. But, as you've written yourself, "the book says that the current year is 1492 DR, which would lead you to believe Baldur's Gate 3 is set two years before Baldur's Gate Descent into Avernus".
We see a mistake with the dates, made by game writers here.
