We, the players with an external view of the whole situation, know that multiple timelines are possible. G'raha does not know, even after bringing about the change in events, whether or not the other timeline still exists.
But again, as far as he knows, the two possible states of the other timeline are (A) it no longer exists, or (B) it exists but everyone is going to die soon and there's nothing that can be done to stop it.
Knowing that it is possible to create a split timeline is not the same thing as knowing that the split timeline will turn out the way you hope it will. But the point of the post-calamity timeline was that it was (allegedly) so bad that any other outcome would be better than the 100% chance of doom they were facing.
You keep presenting it as our character deliberately causing a genocide. I have already covered the fact that the writers (and by extension anyone they regard as good characters) apparently do not share the viewpoint that it is a genocide, so the characters cannot be expected to act as if it were.
Our character simply explained the future that they knew would happen; they did not encourage it to happen. Venat would be on that path whether we said anything or not. We arguably tried to stop it during our time in Elpis, up until the point where it went wrong and also we had the answer we were seeking, so we needed to get back to our present and Venat assured us that she would do what she could to avert tragedy.
It should also be noted that while for gameplay purposes we have easy access to aetherytes and infinite ability to return to Elpis at will, the story makes it quite clear that if this were a more linear game, we would have been there on limited time - probably until Elidibus's power ran out and could no longer keep the portal open. Note that for our subsequent trip to Elpis that leads us to Pandaemonium, we can't just travel there freely but have to reactivate the portal in the Ocular by using the mysterious crystal.
This may be the closest we will come to agreeing, because the Sundering as it happened certainly seems to belong to a Venat who is written as more ambiguous and less "loving mother goddess but in human form" and it feels like they rewrote the rest and forgot to consider that one scene, or else the writers (or different writers/devs) were trying to have it both ways and maintain both "perfect Hydaelyn" and "everyone has their own viewpoint and nobody is entirely right" without recognising their own inconsistency.
Personally I resolve this by just regarding the whole plot as a bit of a mess, ignoring the weirder parts of their philosophy, and crossing fingers that they won't keep being weird when they're no longer trying to write themselves out of a tangled corner of the plot.
Beyond that, I'll talk about the plot as it is written - it's irrelevant whether I personally agree with the proposition that sundering would not count as death, because the characters are acting based on what the writers think is the answer to that question.
Additionally, the writers were never trying to cover all possible angles of the story. By their own previous writing they were locked into crafting a story where the Sundering does happen, Hydaelyn is somehow responsible, and there is no option of it simply not happening. Nor is there an option for the main character to abandon the ongoing world of the story and skip off into the sunset into a different world forever cut off from the setting so far. It's something you can maybe get away with (plot circumstances depending) in a movie or a one-and-done game storyline, but not for an ongoing MMO.
Though in any case, if the whole Sundering scene is a relic from a vetoed "Hydaelyn turns out to be evil" plot, then that just makes me even more inclined to ignore it, assume something else happened instead that lines up with the "yes Hydaelyn really is every bit as good and lovely as she seemed" plot that we did get in every other scene of the story (I'm inclined to assume that Hydaelyn aimed to sunder just Zodiark and the rest was an unforeseen side effect), and move on to discussing the parts of the plot that have no direct relevance to the details of how the Sundering happened.