I think, generally speaking, the Ketheric timeline is just broken. Due to the Troubles, Ketheric’s defeat, and the consequences of apostasy, Aylin simply could not have been imprisoned in the Shadowfell by a Myrkulite necromancer for 100 years. How could Myrkul instruct Balthazar if he was dead during that time? How could Ketheric lose if he was truly immortal, and why does Jaheira say he only returned as immortal a century later? Finally, if Aylin was imprisoned before Ketheric became immortal, why would Shar allow Ketheric to engage in such apostasy with Myrkul? How could Aylin be “tortured” for 100 years if the Soul Cage MUST post-date Ketheric’s defeat and the Shadowcurse?
Concerning Durge and Bhaal, Larian is screwing around with the pre-established timeline. Bhaal definitely died during the Troubles in the 1300s, having had many children before his death to ensure his return. One of these children was Gorion’s Ward. While Larian successfully retconned Abdul Adrian, it seems pretty immutable that Bhaal only returns after the death of Gorion’s Ward, which, canonically speaking, is 1482. It’s possible that Larian retconned the return of the Dead Three to 1462 instead of 1482, but this requires Gorion’s Ward to have died 20 years earlier. My overall point here is that Larian is making changes to the canon timeline, but it simply must be the case that the return of Bhaal only occurs after the death of Gorion’s Ward.
I think the reason no one before the Dead Three undertook the Accelerated Grand Design is that, as Jergal points out, the plan is stupid. Bane, Bhaal, and Myrkul are known for being crafty, cunning, and manipulative, but their plans often blow up in their faces because they are arrogant, shortsighted, and recklessly foolish. Myrkul and Bane were the ones who started the Troubles in the first place when they tried to steal the Tablets of Fate from Ao. Jergal notes that the other Gods (and devils!) definitely noticed the disappearance of souls and worship, and even after the Second Sundering, would find ways to intervene. Mystra, Shar, Selune, and Jergal should not be allowed to intervene by Second Sundering rules, yet clearly they find a way due to the dire nature of the situation. Jergal especially rarely intervenes in anything overtly for any reason, yet he springs into action immediately; screwing with souls and the nature of the universe and fate PISSED HIM OFF. Mephistopheles, evil though he may be, is still a lawful sort who would not take such revolutionary risks.
The Revenge Against Fate motive is more a theory of mine than something directly confirmed. The Dead Three don’t need to rule the world (except Bane), and already gave up a large part of their divine powers during the Sundering. I reason they want that power back. Given Bane and Myrkul tried to steal the Tablets of Fate from Ao already, and that Ao turned all gods mortal as punishment, I think it makes sense for the Three to want revenge and a path to greater divinity. Illithid and extraplanar capabilities combined with Karsite machinations are the closest approximation to something that can be assembled outside of Overgod control while still remaining in Realmspace.
The Artifact timeline is pretty broken still. It is not clear how it goes from Vlaakith to Emperor to Shadowheart to Emperor to Shadowheart. Clearly the following needs to happen:
1. Emperor steals it from Vlaakith.
2. Shadowheart steals it from Emperor.
3. Emperor steals it from Shadowheart and finds a way inside.
4. Shadowheart somehow gets it back while Emperor is inside and Shadowheart is imprisoned.
Alternatively, the Emperor is the Mindflayer with which the PC locks eyes before being taken out by a brick. Which makes the timeline:
1. Emperor steal Artifact from Vlaakith, is freed but hides this fact as the party can to other True Souls.
2. Emperor tadpoles everyone and plants Artifact on Shadowheart Whom he caught trying to steal it.
3. While the Nautiloid is crashing, Emperor goes into the Artifact and saves everyone.
Concerning Omeluum: I think it is established lore that some Mindflayers are born with arcane abilities from the Weave, and that most colonies kill them on sight. The reason is that a Mindflayer’s meddling with the Weave disrupts that specific Mindflayers domination by the Hive Mind. This is how we get Alhoons and Illithiliches.
