Jack finally travels to the troll's universe after John causes the scratch.
Have to say that I'm still a bit confused as to why Jack needed to obtain the uranium and Wayward Vagabond's ship in order to initiate the scratch.
He didn't. Jack appearifying himself into the trolls' incipisphere was not the scratch. And he needed the Uranium to power the appearifier.
Jack destroys Bilious Slick, ending the kids' universe?
Spades Slick kills Snowman, ending the trolls' universe?
If the universes were destroyed, then how did things keep happening?
The incipisphere (both incipispheres) are outside of the destroyed universes. The only characters of note left in the destroyed universes were Spades Slick, The Demoness, and the Condesce.
Lord English arrives...?
Now this part was confusing. I saw how Doc Scratch called the trolls and kids suckers but the green explosion didn't actually reveal anything, and the universes didn't exactly end, so...
The green explosion created the Green Sun. They're suckers because they were trying to destroy it. I guess. The universes did end.
And I'm still not sure that Gamzee is English.
Protip it's actually Nepeta.