Yeah, something I've generally noticed and become conscious of with the whole Ancient storyline is people piling WAY more meaning and scrutiny onto this singular part of the game's story than it was ever intended to be able to carry. As a result, the entire discussion just... warps and deforms under the weight. Sometimes it's really good to step back and center yourself on what we're actually discussing and the characters and structure in play.
And that is that the Ancient story is ultimately a high-concept mythological origin story, essentially 'this world's fall and rebirth', told in a way that leaves crucial actors in it still on the table for the rest of the story. Around that story it's openly acknowledged that neither side of the conflict was objectively morally right at the time, and open to interpretation as to who did right; however, as characters in that part of the story appeared in other parts of the story later in that story, we do have context that one side of that conflict goes on to do terrible things, while the other largely stays out and lets things come to pass in favor of playing the long game (which you could make a case is 'evil through inaction'). Nobody on either side regrets their part in what happens, although some may lament it.
There is room for complexity and subtlety in this, but it's not in the broad events, which by the end is played pretty big and broad even if we got information out of order; rather, it's in the characters. Emet-Selch is a man who never tells the whole truth--likely even to himself--and whose every word on the subject is constantly shifting in bias and shaped truths, even if he never says anything that's technically 'wrong'. Similarly, Venat-as-Hydaelyn also tells a very shaped and biased version of the truth, although rather than inundating us with information and conveniently leaving out pieces, she instead only puts what's important forward as inscrutable monologues and then hides from any questioning or analysis. But even those aren't exactly very quiet elements; in fact, huge parts of how we read the Ancient story comes from when the characters openly acknowledge that, and investigating for themselves rather than just sticking to what they've been told; that's how we got to Anamnesis, Elpis, and debatably Pandaemonium. (I think there's also something to be said that both Emet and Venat had a bit of a 'leak' in the form of an old spectral friend that doesn't follow their playbook.)
Over all the time of this forum constantly relitigating this argument, and trying to find or entirely invent new evidence or angles to find some other resolution for that initial conflict, can often lead to losing what the game actually put forward. As you said, it's fundamentally not serious, and it saves its complexities for how characters feel rather than what they did.
...it's also, and this should also be something we remind ourselves of, only part of a larger story. The game isn't about how the Ancients faced existential dread that forced their society to make difficult choices; it's about people in the Seventh Astral Era living their lives, trying to make their world better, solving as many problems as they can find, and occasionally fishing off a bridge into sand dunes at very weird hours trying to catch a malicious egg. The game didn't have much time to give to this story, so as a result, there's not exactly volumes to write about it, there's only so much room for nuance, detail and complexity. The Ancient story is hardly unique in this, either; if in some alternate universe this forum was instead spending all this time arguing about the legitimacy of the Hannish government, it would also be facing this problem.