Let me tell you a parable, about an ant climbing a building. As it advances, it feels different types of textures, lights, temperatures etc. When it climbs a window it feels hot and shiny, but when it passes a concrete wall if feels dark and cold. For this ant the sequence of the changes is random and sequential, it experiences them over time as it climbs up. So if it was writing his "history" it would write "I've been through tough dark times and times of light and joy".
But for us, seeing the building from aside, the order of floors is very clear, we can see when the ant approaches those dark or good times. So for us, its "history" would be clearly predicted, moreover, it is seen for us as a whole, instead of disjointed fragments.
This idea of a whole 4-dimensional creation (time-space), that's unfolding in time for 3-dimensional us, with all historical events in place is echoed a lot in numerous Midrashim, Meforshim and Kabbalic books.
For example, prophecy, or Ruach HaKodesh, i.g. "seeing the future" can be easily explained by this perception, as in Rashi that brings Gemorah (Meggilah 16b) on Bereshit 45: "וַיִּפֹּל עַל צַוְּארֵי בִנְיָמִן אָחִיו וַיֵּבְךְּ וּבִנְיָמִן בָּכָה עַל צַוָּארָיו" - he wept on two Beit Hamikdashot the are destined to be destructed. There are many interpretations of this kind that vents had influenced not only forward but also backward in time.
THe 4 Galuyot were also destined (as many interpretations suggest).
Another aid is the Mishna in Avot "הכל צפוי והרשות נתונה" - "Everything is seen [at once]". From this point of view, there could be no questions of "what if" as all history is destined from the moment of the Creation.
Concerning Purim - just as the destruction of the first Temple was destined and so the Babylon exile, Purim was necessary to fulfill the establishment of the second Temple, and it was destined just as all other events.