G'raha focused storyline!

[Iscah]

But...how does the Black Rose > Eighth Umbral Calamity Timeline fit into Endwalker's plot? I legitimately do not know this, and if anyone would explain it to me I would really appreciate it. [...] Just by the nature of the WoL dying in the Black Rose timeline, doesn't this entirely break the causality loop of that entire branch? Since we can't go back in time to talk to Venat?

This gets asked a lot but there's actually a really simple answer, because the timeline only splits apart at the point where G'raha meddles with it in Shadowbringers. At the time of our visit to Elpis, there is only one timeline, and a WoL from one branch of the split timeline comes to visit.

The WoL from the Eighth Umbral Calamity timeline never visits Elpis and never needs to. The history of the world as they know it includes their future-alternate-timeline self setting things in motion.

All the events form one big unending time loop with no clear triggering incident; they simply all happen. We visit Elpis, setting events in motion. Events follow through all the way to Shadowbringers where the timeline splits, with events in one of those branches paving the way for the other branch to exist simultaneous with it, and the time loop continues unbroken, allowing it to reach the point where we travel to Elpis and close the loop.

Meanwhile, in that other timeline, they get to the events of An Unpromised Future. Hydaelyn does know about Meteion and can plainly see that the events you described to her have gone off the rails. If you told her about G'raha coming from an alternate timeline, she has probably worked out that she is in it.

Either way, the only reason she kept secrets from you was to keep the time loop on track and ensure that you would not know things "too early" before you get to Elpis. With the loop already broken and the WoL dead, she will need to find a new champion and this time she is free to reveal the entire story to them.

Additionally, since the WoL died in that timeline, they almost certainly did not defeat Emet-Selch or Elidibus, so those two are still running the Ascian agenda (making plans for the Ninth Calamity no doubt), Fandaniel is still in check and Zodiark is still shielding the planet from Meteion's influence. All things considered, the situation is entirely different to the Source's and could be taken in any direction you narratively want it to go.