First off, I think you may be overreacting to the vague stuff from WotC about the context. They make mistakes but they aren’t idiots.
Murder in Baldur’s Gate was a free adventure which, as you say, no one played (really just 5E testers and early adopters and not most of those even). WotC aren’t cross-marketing in some serious way here. You will not be required to read it or the more recent adventure any more than you were required to be up to date with the 2E FR Sword Coast for BG1. You talk about “full context” but unless you were a 2E AD&D Dungeon Master running an FR game set on the Sword Coast (I was, oddly enough), you didn’t have that before.
Larian are not going to write a game which relies on you reading all that either.
Re hype and nostalgia, I do not think that was a “marketing wonk” decision. From what I know of WotC and from what Larian has said and that Obsidian already tried to get a BG3 made a few years ago (not the Black Dog one it seems) as mentioned in a Larian video IIRC, I think this has long been a WotC management goal, and WotC’s management are mostly gamers. Making it congruent with current FR lore is also well outside of how marketing works at WotC.
Whether it will turn out to be a good idea is another question.
Re risk and threading the needle, I guess I don’t see it as so hard. Personally I think that they need to do two things:
Make it so it was indeed Abdel Adrian who died in Murder in Baldur’s Gate, but make him be just one of the Bhaalspawn, not the one who crossed Jon Irenicus.
Make it so the one who crossed Irenicus disappeared into the mists of history, went planewalking, got on a spelljammer or whatever.
Then just avoid referring to BG1 and 2 much.
Alternatively they could take the Bioware route, and create a history builder, wherein you pick some facts about your Bhaalspawn (with an optional default for people new to the series) and then the game uses those. If they want to include the longer lived BG1 and 2 people that’s the way to go.
For me the big issue isn’t the connections to BG1 and 2, it’s whether they can craft a satisfying game that feels like 5E D&D in a good way, and provides an epic adventure.
My suggestion to you is just chill and wait and see. You’re fretting right now and it is understandable, as a serious BG fan, that you would be! You have my sympathy. But just breathe out, and forget the game exists until we have more details. It is not coming out in 2019, despite misleadingly being suggested to be a Stadia launch title by Google. So there will be a while yet!
Edit: Man writing about the history builder made me think “remember when Bioware were cool?” and that hurt it wasn’t even that long ago, just 2015.