Reposting some stuff I've said in that thread:
Re: Eternum - Everything about it basically screams Astral Plane or the Cariverse version of it, from how you actually access it ((projection/out-of-body experience induced via the old-fashioned way (dreams, hallucinogenic substances, near-death experiences) or the Ulysses way (nerve implant, which likely just induces the body to go to a deep sleep and thus triggering OOB is always 100%)) to the fact that the dead themselves exist within it and to William's infodump stating that some people mistake it for the afterlife (which it isn't, as it's simply a plane of existence that the dead pass through on their way to the afterlife...or at least, that's SUPPOSED to be how it goes, but obviously something has gone horribly wrong with the order of things for them to be trapped there for a very long while).
Re: The Founder and the NPCs - Just a theory, but if we believe William's claims about the dead then it's possible that the state of Eternum - that being a plane where the dead are trapped rather than the pathway that the dead pass through as intended - has already been that way long before the Founder even entered the picture. Otherwise the idea of the dead being "drawn from the pages of history" wouldn't hold if the closing of the Gates of Acheron (which is likely the incident that led to the dead being trapped in Eternum) was fairly recent. Unless of course you argue that the Founder is older than we assume him to be...which would be weird considering that William claims him to be "just a man" whose overall goal is godhood and yet he already does godlike things, which would be contradictory.
"But he chained Idriel, so it's likely that he's powerful enough to disrupt the natural order and enslave the dead." Well, do we know that he did that? Because it could also be the case that Idriel was already chained before the Founder came into the picture, and is simply being exploited thanks to her current state. In fact, that's probably why Ulysses managed to set up fast enough, as the harder stuff (the dead in stasis and can be exploited as NPCs, Idriel imprisoned and can be exploited for her mass teleportation abilities via the Nexus) have already been handed to them on a silver platter. It's also basically a double-edged sword as they have limited understanding of what the worlds (and denizens) of Eternum are actually capable of, which is obvious considering their difficulty in containing the Xeno outbreak and stomping out the Syndicate...and (likely) not being aware that the person in charge of ferrying players across the various worlds is actively working against them.