I'm here to answer one of my own questions briefly, in case anyone else was interested in this.

The writers of Wulbren and Barcus:

https://twitter.com/quirkiltons/status/1759342127106977877

Rachel Quirke says she is "the writer of a bunch of Wulbren's stuff" in this tweet, and she mentions in the replies that Sarah Baylus wrote Barcus.

Apparently Sarah Baylus was also the one who wrote Wulbren's Spattered Diary, which is one of the few things people in the Barcus & Wulbren spaces use to discuss his possible complexity. I thought aspects of this sideplot were inconsistent and unfinished before, but it seems like this was the intended outcome. (And there may not always be discussion between the writers to include such things?)

More than likely, someone else also wrote part of Wulbren's dialogue or story, but I don't know who. Rachel Quirke is Auntie Ethel's writer, but I recall D.E. Chaudron mentioned writing her Vicious Mockery lines: https://twitter.com/DeathMeetAuthor/status/1711446814325444688 so this is how they operate!

Slight frustration at no one correcting the people who assume Gondians are a race and not a religion in the writer's tweet about him while still claiming he has layers... He was voted the most hated character in the game. He doesn't seem complicated to anyone but the twenty people online who are (or were, I admit reluctantly) trying to make their own sense of his irrational and inconsistent behavior. I guess the human Gondians also didn't live in anyone else's playthrough?

I finally played through Baldur's Gate: Enhanced Edition, though I've yet to play BG2. Gondians do exist in that game, and you can even visit the Hall of Wonders and High House of Wonders! I know there are svirfneblin in the Underdark in BG2, but not sure about the mention of their banishment from the city yet.

In older Forgotten Realms sources and Dragon articles, Gaerdal Ironhand didn't even exist. I'm not sure which text was the first to introduce him into their pantheon, but the Ironhand/Gondian conflict is definitely entirely new to BG3.

So it appears there are no secret missing answers that make the way Wulbren acted make more sense, if anyone else came to this post hoping for that. Just what we already have in the game, which, now that I know what I know, is probably just hatred that Gondian gnomes left Ironhand for Gond, possible economic class-related frustrations, and what he believes Gondians did to his own family (although it's likely not how it actually happened. Wulbren seems to be from a very mentally unwell family. Give all the deep gnomes therapy).

I like the idea of runepowder being introduced to the Realms because it's an even bigger threat and could change who holds power in this universe, but it doesn't seem like more can come from it with the way it was resolved in BG3, and I heard there are no plans to make a BG4, at least from Larian.