Honest question, why are you trying to include lore from other media (books, even other games) into BG3s timeline?
It's a rule known to all D&D players: Every DM creates their own world. They might use a known setting, they might use characters or other elements from a setting, but at the end of the day they decide how the multiverse works, what Gods are / are allowed to do / how they work. How magic works (if it even exists) and the timeline are 100% up to the creator.
That said, there are definitely things to discuss about BG3s timeline in particular, but anything from any other sources (including BG1 and 2) should be ignored imo. They included elements from BG1 and 2 but in LARIANS world they can act however Larian decides and whenever Larian decides. Bhaal has become active when Larian says so, etc.
In short, the only lore that matters for BG3 is BG3 lore. Just like you don't go on about how Elminster SHOULD BE according to x source when sitting at a DMs table.
Baldur's Gate isn't some "random" game, nor does ANY of the players get to be Dungeon Masters, ever. Players are, without exceptions, participants of the game and not the authors of it. Even if Baldur's Gate1, 2 and 3 exist in their own separate universe, away from anything and everything WOTC and Larian made before, Baldur's Gate still has it's own established universe, rules and plots that you can't break away from. You can't have on one end say "The Dead Three will be ressurected and unite again upon the death of the last Bhaalspawn (which is Gorion's Ward in 1482, canonically)" but then have Bhaal be active many decades before said event would occur. Here is the thing - ultimately, it doesn't change much. They could just have the entire plot delayed by 30 years and the plot would work just as well, but inconsistencies like this tend to build up over time and eventually degrade what could otherwise be a fantastic plot.
Imagine a timeline with what I just described:
1482-1512
- Gorion's Ward dies, the Dead Three are ressurected.
- The Three start rebuilding themselves, completely solo at first. Durge is created, Orin is born and the Temple of Bhaal is inhabited again.
- Balthazar, while not acting as Chosen of Myrkul, begins carrying out His most important tasks - one example being searching for a prime candidate to make Chosen.
- Bane is at a complete loss initially, until Gortash stumbles into Him.
- Contact between the Three are scarce, but slowly develops.
1512
-Gortash, now Chosen of Bane is tasked with retrieveing the Crown of Karsus.
-He meets the now Chosen of Bhaal, Durge, and the two develop their relationship.
-Later down the line, Balthazar finally finds the perfect candidate to be Chosen of Myrkul - Ketheric. Regardless if the Absolute plot was going to happen or not, but Ketheric's ascendance is a well timed convenience for the Dead Three.
-Everything else happens.
1522
-The events of Baldur's Gate 3 take place.
Nothing changes here, but now, there isn't a chronological disturbance of events that you have to retcon for in future installments. And this is just one issue of many, and as said, small issues - even if ultimately nitpicks - tend to add up and ends up ruining the immersion.
Let me ask you this: have you ever had a DM in any DnD game that you ever played who just completely ruined your immersion? They set up what is otherwise a brilliant thriller, only to make some absolutely insane blunders in the presentation of certain characters or such that ends up ruining the entire plot? It does not matter that that DnD universe is "temporary" and "custom" - your immersion was still broken, and the damage has been already dealt.
