Retcon is short for Retroactive Continuity.  This is the process of changing facts that were already stated.  John is going back and changing parts of the story that were told years ago. And the changes that he is having an effect on how the story will continue to be told, such as Vriska still being alive.

Because Gamzee still killed them.

No. The trolls session needed to be doomed and scratched so that the post-scratch trolls could exist and make the pre-scratch trolls so that they could play the game in the first place.  Whereas John made the kids before hand so they were already made and can still play without having to worry about it.

Gamzee set up their living place and provided food for them so that Caliborn could grow up and become Lord English.  He was also the one that chained them to the sarswapagus 

a year

Dave, Jade, Aradia, and Kanaya are all still alive.

Not in canon

It’s never said she is in canon. but it also never said she wasn’t. so… to each their own.

Vriska likes bright red shoes.
Source

In Act 2 John picked up the bunnykind strife specibus
Source