...the person literally right above this post spent the past two pages trying to argue that it's okay for us to leave the modern world to die so that we can instead try to save the Ancients. (And frankly, this is already the most kindness I've ever seen readheadturk offer the sundered people.) Vyreus tried to attack Cilia's arguments for being anti-perfection. If you can't read that far, I'm not going to help you any further, you clearly aren't listening.
And of course the Ancients are all pretty people who people fall for. Have you been on the internet?
A 'message in a bottle' doesn't work, specifically because of the timeline split you're trying to make happen. The message can't get to the Scions, if the timestream doesn't flow to them. And that's even putting aside that you can't make a message that'd get to them anyway; the Calamities grind everything to dust. The only message that ever got there was Erich's memory crystal, and that thing was garbled to hell and only arrived in a weird place the Scions weren't looking, so no, that's a terrible way to get an important message out.
So what the WoL would have to do in your (very poorly considered) scenario is basically to pick a point in the Elpis trip to stop, go back to the Scions, tell them what they've learned and then go back. Okay, I'll ride this and see where this goes. To do this, we need to figure out at what point does the WoL have actionable information, that the Scions can use?
After talking to Elidibus, learning that Elpis was a place run by Fandaniel? Firmly 'that's nice, dear'-level information, that doesn't help.
After learning Emet was there? Of course Emet knew Fandaniel, that's not news.
On meeting Hermes and Meteion? At this point, they're innocent bystanders; not useful.
On meeting Venat? That's almost an actionable lead, it proves Hydaelyn might know something, but not one of direct usefulness.
When Meteion starts getting the report back? At best you've found the likely culprit, but that doesn't help. Getting a name to curse as you die doesn't avoid dying.
Really, the only choice is after Ktisis, when the WoL returns anyway; it's in Ktisis that we learn that Meteion is definitely the culprit, as well as why, and that Venat tagged her and so probably knows her whereabouts. So essentially we're looking at a scenario where the WoL completes all of Elpis, bails out the Scions in Garlemald (at least, I hope; your idea better not be 'leave Krile a memo'), and then tell them 'hey, ask Hydaelyn, I'm gonna go help her back in the Before Times'.
Because you realize that's what they would do, right? The WoL can't save this independently, and wouldn't want to. Venat doesn't just know everything actionable here save for maybe 'what it's like to die to the End of Days',which I probably wouldn't call helpful; she's also better known in the Ancient world, better equipped to either figure out a solution or to find people to help her--which is, y'know, what she did. The WoL is a blind idiot in this context, and like in every context in which they're a blind idiot (i.e. every expansion, often more than once), they'll defer to someone who knows the place better.
And Venat didn't want our help; she wanted us to go back and help the Scions. Because she knew which end of the timeline we'd actually be useful in.