The general canon rules for FR modules is that Adventurers do not ultimately change the status quo of the situation - so if a Lich is trying to bath the world in a life extinguishing fog - they are presumed to succeed in stopping it. If a devil drags a city into the hells, they are presumed to see it returned... however, if graa'zt is involved in a demonic soul claiming plot, the adventurers are presume to stop it, but they are NOT presumed kill or otherwise dethrone him in the process...
So, what this means for the "general rules" of module canon... is that Elturel was rescued, but Zariel, unfortunately, is not presumed to have been redeemed.
