I don't think at this point Descent into Avernus is prequel, if anything they'll be running concurrently, I'm not too worried about the dates that appear in destroyed tollhouse, I'm just basing this on where the pieces are on the board right now.
We know the Tieflings are from Elturel and Zevlor is a Hellrider, they went into Avernus during the Descent I assumed they'd returned from Avernus because they've said Descent was a prequel but actually I don't know that, they could have been outside the city when it's taken I don't remember what Zevlor says on the matter and are now back (or are going to BG after Elturel was taken), we also know that recently Duke Ravenguard was kidnapped by Absolute Drow, a conspicuously familiar event.
If it isn't a prequel, why the successful check in-game mentions it in the past tense?
How those refugees were able to leave Elturel at all (if it's still chained in Avernus, events running concurrently)?
I'm pretty sure these lines in-game have to be re-looked into by Larian devs in order to give out a finished picture for the player, unlike right now.
