That's why I said that the theoretical cut content for Karlach was explanation why Gondians can't help, something like you said. The story would be better but wouldn't change in the end and since Larian was short on time, they couldn't implement it or prioritized other things. Also, isn't the point of Avernus ending (especially after patch 2) precisely what you said? Heroic/desperate attempt to set things straight, I think if you're playing Karlach as origin and romance Minthara, she says she'll go there with you because that's where the fix is so it kinda confirms that going there is looking for cure. If the epilogues will be added, they'll propably explain it in more details
Well, one, "We can fix the engine but we can't install it here so that's why you need to go back to Avernus WITH a fix in hand just to find a way to implement it in the place where they already did it once" is miles more hopeful than "We can't fix the engine, you have no plan or cure, you're going to Avernus in a desperate attempt to live more and it all hinges on a baseless hope that we'll maybe find something". What we have now is the latter, what I was talking about is the former, and they're very different tonally.
(To be clear here, I am not talking about a playable post-game DLC even, I'm talking about the same open epilogue we have now just holding way more weight and reason.)
Secondly, "This likely was a part of the quest but they ran out of time and dropped it to prioritize other things" may be an explanation, but hardly an excuse. Especially in light of the wild difference in content amount between certain characters. If there was such a lack of time (which yes does happen in development, yes, and devs shouldn't have to crunch), maybe Astarion could use one or two scenes less in favor of characters like Karlach or Wyll?
This is not a dig at Astarion btw, he's just the most glaring example of what all origin characters ideally should've had. And if there wasn't time to give them all this treatment, then it's not entirely unreasonable of us as players to complain that the attention got split unevenly and resulted in what feels like quest holes. Because let's be honest, if something was indeed planned but dropped (especially if it's just Gondians telling us "no", which would be like two voicelines), then it was obviously something important - otherwise we wouldn't have so many people convinced they missed a part of the quest or messed it up.
Whatever it was (if it was anything at all, of course, but we all agree that the present Act 3 hints are suspicious), it's absence is kinda glaring in the pacing/reasoning, which is why we're politely hoping that something could be added/restored eventually.
(p.s. Honestly if it was just Gondians/Ironhands telling us to scram I'd say that would be patched in by now)