After creating 2 alternate timelines (one where Ron marries Padma Patil and one where Voldemort ruled the Wizarding world), Scorpius, with the help of the alternate Snape, Ron and Hermione, travelled to the First Task and stopped the other Scorpius interfering with the task.
The then travelled to an timeline that was identical to the timeline they left.
What happened in THIS timeline? Did Cedric STILL become a Death Eater over losing just the 2nd Task? Why was Umbridge still Headmistress?
In short - what events led to the 3rd alternate timeline in Cursed Child?