Kelly’s answer to “In the end, on the original timeline, the chair is already built when Barry is about to meet Helena…” > Likes and Comments

Oh my god. You broke my brain. Has anyone else not noticed this?! I need answers now.

I'm pasting my answer from another question that explains why I think Barry returned to the original timeline:

"I recalled Barry's first experience with FMS and having two sets of memories. He's just met with the Behrmans who had turned him away and he's halfway through eating huevos rancheros in a diner in Montauk before boarding a train back to the city. He knows he's being tailed by someone who is later revealed to be Vince, just as another set of memories rolls in, in which he never encounters Ann Voss Peters on the roof of the Poe Building, then looks up the police report of the attempted suicide of Franny Behrman or travels to meet Joe and Franny in Montauk and he's at his apartment in Washington Heights where he's watching a Knicks game instead of eating at a diner in Montauk. He has two different recollections of his past few days leading up until a few minutes prior to arrival of the alternate memories and he fears he has FMS.

Now when Ann Voss Peters recounts her life as Ann Behrman to Barry on Nov 2nd, 2018, she says she gained her memories of that life a month prior, i.e. October 2018 and in that life, Joe's first wife Franny had killed herself by throwing herself off the Poe Building 15 years prior.

If I'm infering correctly, in the original timeline, Joe's wife Franny committed suicide sometime in 2003 and he eventually meets and starts a life with Ann in Vermont where they run a landscaping business together and have a son named Sam. Although they're happy, Joe never gets over his guilt of being unable to help Franny and losing her. So when he finds a chance for a 'do-over' with Slade's chair, sometime in October 2018, he has Slade send him back in his memory to 2003 to save Franny.

Barry gains his memories of never meeting Ann and Joe Behrman on November 5, 2018 at the diner in Montauk. Helena and Slade on two separate occasions tell Barry that Slade killed her on the night of November 5, 2018. Which leads me to believe the memories of never meeting Ann and Joe Behrman that Barry gains in the Montauk Diner are Barry's memories of the original timeline, in which he might have also remembered a world that was unaffected by FMS. So in the end, Barry attempts and succeeds in traveling to a dead memory, a recollection we read about early in the book and stops Slade from killing Helena, thereby resetting all timelines."

So when Barry returns to the original timeline, the first test subject had went back in his memory and died in the car wreck. And no one but Slade realizes the full potential of the chair from the test subject's death. So Barry kills Slade before Slade can take control of the chair. If Barry had gone back earlier in the original timeline, say before the first test subject's death in the chair, he could've perhaps prevented Helena and Slade from testing out the chair in the first place.

But he didn't, he chose to return on Nov 4 because the events of the day - Barry and his ex-wife reminiscing about their dead daughter in a cafe - in the initial pages of the book had become a dead memory by Barry being forcefully sent back to his past to save his daughter. At the end of the book, by traveling back to a dead memory and killing Slade before he killed Helena and started off the chain of events once again, he reset all the timelines. Because of the abrupt ending, we can only infer that Barry did his best to convince Helena to destroy the chair or did it himself either without her consent or knowledge. I don't think he would've tried to save the first test subject, as that would once again bring about the problem of alternate timelines. And Barry's arrival on Nov 4, 2018 in the end of the book, and his actions afterwards would still be a part of the original timeline, the test subject's death being chalked up as an unfortunate accident and with the chair destroyed and Slade eliminated, the world would go on without ever knowing about FMS and alternate timelines, and there would be no more branches from this timeline.

I know it is a little bit late but I have just finished reading that stunning novel and your comment left me thinking.. you are partially right about that..he didn't return to the very original timeline..he returned to the first alternate timeline created by jon jordan return to his wife's death memory.however, it is not the point on which the technique depends..when slade demonstrated it to barry, slade didn't return to the original timeline himself to erase Helena's memory about the the technique they discovered..the point relies upon using a false memory that correlates to an event before the one you intend to change.. that's what barry did..he used a false memory to return before anyone could understand the powers of the chair (except for slade of course) and murdered him to regain order to the timeline (first alternate). sorry for the lengthy explanation