Sorry if I missed some important plot points, my memory's still a bit fuzzy on the whole retcon thing, but why didn't Terezi make John change things so that none of the trolls die? Or at least the trolls that died from Gamzee's hand.

Terezi can only hone in on points of the past she knows very well, so John may not have been able to get to the appropriate time even if she tried. That’s mechanically, at least. The biggest issue with Terezi sending John on a retcon quest, ties more to Fate and Responsibility.

John doesn’t want to change anything too critical, lest everything become unrecognizeable when he gets to the Session. The changes have to be precise and surgical as to not create a completely different reality where everything is different and confusing. On top of that, some deaths are necessary for Paradox Space to continue as we know it has to- Equius’ specifically, as he needs to become a Sprite. Terezi doesn’t know what stuff she’s not involved with will lead to, but she knows of a few things that have been haunting and torturing her, that she feels could give them a bit of an advantage- IE: Gamzee being caught, Vriska being alive. Later, with the exception of the Seadwellers, every other Troll is brought back in some capacity. Eridan kinda deserved that death, and Feferi is the only one who gets the shaft- Buuuut she kinda made the Bubbles and likes the Horrorterrors more than anyone else, so she’s not that bad.