the Long Night, ended just over a hundred years ago (page 180), and the Zenith arrived 66 years ago (page 248) assuming the current year is 61 CC), so the Long Night was maybe 50 years long. That does not seem nearly enough time for people to forget the advanced technology, for cities and outposts to fall, and civilisation to recede!
And, at the same time, the Nazareem's Sacrifice cult, that was wiped out over a hundred years ago, is still around as sleeper cells in ways that the First Horizon feel confident of being able to contact. Are these multi-generational sleeper cells?
Finally, what exactly did the less than 100,000 people on the Zenith, suffering memory loss and madness from low-tech cryo-sleep, do to kickstart trade and civilisation in the Third Horizon?
Hi, hope I don't come off as too preachy or condecending, just sharing my take on the game and trying to help.
First: your logic is nice but flawed.
The zenith arrived ca 61cc ago, and the long night was ca 100cc, this means that in the current game timeline, players are playing in a time period roughly 161cc since the portal wars. (Also , the period between the arrival of the Zenith and the start of the construction of Coriolis station is a bit vague, so you are free to imagine that maybe it took some years more than what's written in the book)
Further, the timekeeping standard of CC is just 61cc old, meaning that any event beyond that is vague and imprecise, as each system had their own calendar before.
(Skipping the Nazareem question to last.)
Secondly, you are making some assumptions about the Zenith and it's crew that makes you paint yourself into a corner: where does it say that 'Everybody' onboard the Zenith suffered from memory loss and madness? Why are you so sure that the cry-sleep was lower tech than what exists in the third horizon today?
Regardless, the answer to your question is that 'what the Zenithians brought that changed the Thrird Horizon ushering in an era of commerce and exploration' was their technology and understanding of portals that many firstcomers had lost. To use a Rome as a historical comparison: "all they did was build roads".
Lastly, you are not the first to wrangle with the timeline and question the logic of the history of the Third Horizon the way it is presented. I would recommend that you remember the following:
1. This games genre is Science Fantasy, so it's adherance to logic, math and science is only as far as it helps it tell its story of courtesans with electric fingertips, warriors wielding thermal rifles, smugglers with cybernetic eyes, and arabic mysteries of wonder and horror in a glaxy clustered littered with secrets, some of them older than even the Icons.
2. For any story to work you have to suspend some disbelief and buy into the premise the story (and storyteller) weaves.
You question how a cult can be wiped out and still ressurface years later?
With stasis pod tech and so much uncharted areas in the Third Horizon I ask you how can they not be?
(check out the Talons of the Court of the Owls in the batman comics some years back for inspiration)
3. All the info we have is presented in a "player-facing way", by that I mean that all the lore in the game is written from the perspective of an individual living in the Third Horizon, not for us - the readers. Which means it can be and most certainly is partially wrong in many places. The game doesn't tell the GM anything about "the truth" of the Icons, the 2 other Horizons, the location of Al-Adrah, but rather tell them that they have carte blanche to do whatever they want to the setting, its their game after all.
In my case I have decided that the Symmetry is a messed up teocratic hive mind (almost like the borg from star trek) ruled by mystics, and that the First Horizon is more akin to Terrans from Star Trek: discovery, I have placed the grave of the Martyr in Uharu, I have explained at least two "portal builder ruins", and at lest one Humanite type as actually being abandoned Symmetry Tech, and so on and so on ...