What's the point with that quite lackluster story in Endwalker?

[Cassar]

As for Elidibus' comments about making changes; they could just as easily be interpreted as a warning. For if you were to make changes in such a way as to prevent the sundering or otherwise change any circumstances leading up to the present, you may not have a world to return to at all. Or if you did, it might not be yours. ....Or you might simply cease to exist, which would by default undo whatever you did. Luckily, the predestination paradox is there to prevent you from ruining everything. Then we've the previously discussed option of him simply not realizing Emet-Selch was going to reinforce your aether and thereby give you the ability to meaningfully interact with the world of the past.

So in your opinion (and anyone else's who wants to reply) what do you think would happen if the WoL went insano mode and, let's say, killed Meteion then and there. Do you think we would've created a different branch in timeline (similar to Graha's timeline) and therefore creating a third version of the WoL, or do you think we would come back to "our" reality only to realize it's completely different (the world was never sundered perhaps). Or would it be something else?
I don't know if there is any indication in the lore that would point to which of these would be the "right" option, which is why I'm asking what you think personally.

I like to believe that we would've created a separate timeline, one where the WoL is stranded in Elpis unable to return, for they have no way to travel between timelines.

[Absimiliard]

So in your opinion (and anyone else's who wants to reply) what do you think would happen if the WoL went insano mode and, let's say, killed Meteion then and there. Do you think we would've created a different branch in timeline (similar to Graha's timeline) and therefore creating a third version of the WoL, or do you think we would come back to "our" reality only to realize it's completely different (the world was never sundered perhaps). Or would it be something else?
I don't know if there is any indication in the lore that would point to which of these would be the "right" option, which is why I'm asking what you think personally.

I like to believe that we would've created a separate timeline, one where the WoL is stranded in Elpis unable to return, for they have no way to travel between timelines.

That depends on what kind of system the Final Fantasy XIV universe operates on. Thus far we've seen one example of multiverse theory in action, and some of what Venat says about a convergence (which Lauront already quoted, I believe) could be taken to imply the potential for another to have formed already if not for mitigating circumstances. Based on this, I would tend to say we've got two a few options, but some I believe are more likely than others.

1.) The convergence Venat speaks of causes the WoL's sudden slaying of Meteion to, in essence, destroy the timeline from whence they came. They cease to exist as a result, their presence in Elpis never occurs to begin with, and the Final Days proceed exactly as they would have without the casual loop to ensure the birth of Hydaelyn and subsequent sundering. Whether this means the loop reasserts itself due to Venat reaching those conclusions on her own, the Ancients manage to solve the problem in the time Zodiark gives them, or the Endsinger ultimately destroys it all is anyone's guess.

2.) The convergence functions as above, except the WoL continues existing as a paradox unto themselves. Everything the WoL knew and loved is erased the instant Meteion dies, but they are protected by the paradox and retain full memory of the world from whence they came. From here it's anyone's guess as to how events would play out, but I suspect Venat, armed with knowledge from a future that no longer exists, would make the same choices. Well, assuming the Final Days could come at all.

3.) Significant deviation from events required to maintain the casual loop prevents any sort of convergence, and a new timeline is split off from the old at the precise point Meteion bites the dust. Events proceed as with option 2, save that the 8UC (original) and derivative (the WoL's point of origin) timelines continue to exist. One can only speculate as to whether or not the WoL could return to the future, as this would presumably require the new timeline to have had events play out in a manner conducive to a corridor between present and past having been opened. A corridor leading back to the WoL's point of origin is highly unlikely.

In any case, this is getting really far into headcanon, which I've never been a big fan of.