I've been lurking and reading the thread and would like to point out some things that nobody seems to have mentioned yet.

There is one part in the sequence of events that nobody has really talked about: The time after Emet-Selch, Hythlodaeus and Hermes returned to Amaurot but before Hermes was appointed as Fandaniel. If Venat revealed the truth at that time, there would be more variables at play.

1) The previous Fandaniel would still be alive as he only returned to the star after Hermes was appointed, we can assume that he's at least as capable as Hermes.
2) There's still some time before The Final Days is meant to happen, giving them some time to plan countermeasures after going through Hermes' research about Meteion. We have to remember that it's canon that Hermes was the first one to notice the thinning of aether, nothing says that he's the only one capable of noticing.
3) If a countermeasure is implemented that doesn't involve the mass sacrifice of ancient lives, Azem would still be fully on board with the convocation.

These 3 variables from that point in time might cause quite a large difference in outcome.

Side note: I find it weird that people are in a speculation thread trying to shut down other people's speculation because their own headcanon says the characters wouldn't be up to the task. Why not just let people have their fun?