The thing is that if Graha always called us and the scions then Tataru must have also sent out Estinien to Garlemald and Varis would have been killed because Zenos went to him because we were not there. (Otherwise it would mean that characters suddenly act different under the same circumstances without reasons) Zenos does not want to use black rose and Gaius and his followers destroyed quite a bit of it. At the same time at that point Eorzea was at a stalemate thus the use of black rose was further away.
How would the other timeline then even happen? For me it makes sense that there was never a call, we never saved the first, the first never had a tower thus got lost to stagnation under Vauthry, and our side started to win. Seemingly Varis was still alive and black rose was used.
We could of course assume that the information we have are faulty. That people simply did not know the full things that happened in the past but I am not sure if it can be truly used as an argument.
Its true that its not said that we were at the battle but Uriangers does say that the weapon (black rose) stopped with nobody including us. (And then shows all the scions being death with the twins directly on our side) If we died on the first why tell us that we died by the weapon? Of course it could again be just rumors and that people started to believe that we died by it too but isnt it just more simpel in a story telling way that in the bad future nobody told us about the first, nobody called us and thus created all the new possibilites and thus we just died on the source.
So for me it makes the story much easier to understand if we had one timeline and it would have ended badly. But (in a way to prove the Ascians wrong too) the people of the future, inspired by our deeds, made the impossible possible and created a way to travel back to stop this from happening. And the change was Graha Tia and his calls to the first. And with the calls of the scions and us we set the change in motion.
Of course there is the question on what happened with the old timeline. If you say that our decisions split it then we should have a massive amount of such timelines. (Its like the strange Zelda timeline where one timeline with its games only exist if Link dies in a certain battle against Ganondorf in OoT. Even though he could have died more often too so there should be more right?)
Its not impossible that they go that way, especially with the Exarch wondering on why he still exists, but maybe he is simply a paradox in some way. I just wish that its not that complicated. Thats why I believe that there is just one line and we simply for the first time changed it.