Giorno getting a Requiem stand isn't random. Giorno actually tries to activate GER
before the final fight, and
fails.
It's the same thing as Kira. Kira acquires Bite the Dust in his moment of ultimate despair, when the arrow
chooses to pierce him. The arrow recognizes Kira's ultimate resolve and pierces him.
The arrow refuses to stab both Diavolo and Giorno because they lack real resolve. Diavolo believes he's destined to win, Giorno's motivations are not more complex than becoming a gangstar. It's in his moment of ultimate despair, when everybody's dead, and he's about to be dead, that the arrow chooses to stab him and create GER. It's the resolution of Giorno's character arc.
If the Cinderella arc does get moved up the only thing that gets screwy is now Kira coincidentally finds his way into Aya's salon whereas in the manga he knew about it by keeping tabs on everyone.
You could always say Kira knew about her powers, the same way he knew about the ghost alley.