Wow. This whole post is absolutely incredible. It makes me so happy to see someone recognize and deeply analyze how crucial retcon was to the story and characters arcs and not just "Bring Vriska back".

It was so nice to see John get to be in the spotlight again and prove his competence and strength as a leader. Watching him lock in like you said by responsibly using his new powers and encouraging Roxy to not give up was just such a great moment. Also, that part of retcon made me fall in love with roxygen and I love their relationship so much.

Terezi really getting her chance to shine is really the highlight of retcon for me though. She has always been my favorite, and I think the panel of her pointing down to John telling him to "F1X TH1S" might be one of if not my favorite in the whole comic. After spending thousands of pages just watching her spiral until she truly hits rock bottom, watching her switch to the most determined we've ever seen her and ready to pull it together is just so satisfying. She really gets to show how much of a badass she is and gets shit done. All of this being followed up with the passwords absolutely blew my mind the first time I read it.

I don't know much I can say about the other characters that you didn't already say but obviously a huge part of the post-retcon timeline is that Vriska is alive. I know a lot of people saw retcon as "everyone died just so Vriska could be brought back" and while I obviously don't agree with that sentiment, Vriska remaining alive for the meteor trip was just an obvious overall net positive for everyone. I mostly want to point out particularly in Dave's case though because in the post-retcon timeline he was able to fully come to terms with who he was under the layers of irony he put up as a defense. And we get to see how that changed him as a person in his conversation with Dirk before Collide. Retcon allowed for Dave to be one of the only characters in my opinion to get a fully satisfying character arc by the end of the comic.

Every time I find myself wanting to go back and read a little of Homestuck, I find myself wanting to reread Retcon. I remember reading it for the first time and realizing how much I truly love this comic. There is so much more I could say about it, but you did such a great job summing it up already.