The timeline moved backwards?
That's just...no, there are no words.
I second Ben's motion to jump forward a couple hundred years anyway, as I was under the mistaken impression GW had already done. I do like the idea of adding 39000 years to present day dates for general Conclave stuff, but without the plot device of hundreds of years having passed to muddy the waters, I'm not sure this particular story would work.
As for the taking of liberties, by all means...I still get a warm glow whenever someone mentions one of my characters/locations IC, and the writing so far has been thoroughly enjoyable. One thing I am loathe to point out but probably should, for the sake of continuity - last we saw Secret's Hold, I'm pretty sure Charax literally destroyed the entire planet. That said, I did have a fondness for the old place, and the scene currently going on there has a lot of promise...in which spirit, here are some possible explanations for it still existing:
1) Perhaps Charax only appeared to destroy it, instead merely temporarily scouring it of life/hiding it in the warp.
2) Either he, Amon Dull, Escellon or some other demon made an exact copy of the planet for nefarious purposes yet to be revealed (which amounts to "a demon did it", but it's not as if we've never used that one).
3) It is conceivable that I'm not remembering this right and it was a different planet we nuked.
4) There could have been more than one Secret's Hold, all on obscure deathworlds and made with similar materials (Di Valdi having visited Balkoth and Mentirius there hundreds of years before the Amon Dull Saga really kicked off, they might have moved their primary headquarters in the meantime...or maybe the Hold was always spread across multiple planets, its component parts linked by warp gates...I do quite like that idea, though it would need further development...)
5) It's also possible I'm being too much of a purist. Clearly GW have no qualms about retconning stuff, so maybe we shouldn't be too militant about it ourselves...
As for my own character(s), I'm still at the vague ideas stage, although having a date set in my head now makes things easier. I think I'm going to focus on members/associates/unknowing agents of the Mentirian organisation, which will by now have been decentralised and become an underground, secret handshake kind of a deal. I'm sure there will be an Inquisitor or two in there somewhere, but I'd expect most of the members/puppets of members not to be Inquisition personnel...after all, with a few notable exceptions, the bulk of its members in the early days would have been people unofficially employed by Mentirius, rather than his peers. The Book of Mentirius (which needs a more original name) would have to still exist within their network, acting as a history and manifesto, but officially it would have been listed under "to be burned with extreme prejudice" ever since his death. Which reminds me of another thought I had...
Ben mentioned forbidden manuscripts, their dissemination and suppression, as as a theme, and we seem to have several such floating around IC. There's the data bank in the first post, Taren's testament in the second, Mentirius's own book, and potentially Sargoth's Book of Doom or whatever it was called...although I'd imagine the latter ended up in Charax's care, along with its bearer, after the attack on Delan's Point. What I'm wondering is, are there any others we should consider? It seems to me that if we aren't careful, we could easily end up with a whole hosts of references to "The Book", with different readers making different assumptions about which book is being referred to in any particular post. So far I've been assuming that the data bank that contains the most sensitive/comprehensive information and is therefore the primary plot device, but clearly the others will play a part somewhere.
I've also been pondering the strangeness of Imperial politics...given how totalitarian the state is, you'd expect more straightforward political dissidence to exist, as in dissidence totally unconnected to the influence of Chaos or any particular alien race. I know there's a strong rationale behind the Imperium being so brutal, what with how many enemies it has, but surely plenty of its citizens would disagree, especially given the lack of information many of them would have about the wider galaxy. There can't always be a convenient cult to hand for for the discontented to join up with. Obviously rebellion on a wide scale would be grossly impractical, but some of the more isolated planets must be rife with conspiracies, and they can't ALL be the result of outside influences or malignant religions. Maybe it's that there are so many demons, aliens and untrained psykers around around that no half-decent conspiracy lasts long without being hijacked, but we're talking about a staggering number of planets, each inhabited by billions. If Recongregators within the Inquisition - an organisation theoretically dedicated to preserving the Imperium the way it is - can object to the way things are enough to actively try and change them, then why not some of the common people? Maybe they do, and get so swiftly crushed every time they try without some otherworldly power backing them up that no one bothers to record it...there's probably an obvious answer to all this, but if so, forgive me - it's been a hell of a long time since I read any official fluff.