If I go through the mostly-burned door, the room next to the person I need to rescue is engulfed before I can get my characters into the room.
I wouldn’t worry about the room next to Benryn. I think I went to the trouble of rescuing the chests in there once a couple of patches ago, but there wasn’t anything very good in them so I never bother now.
I do agree that this quest is annoyingly fiddly and unintuitive, but I’ve never failed to complete it (unless I failed the rolls) when going into turn-based mode outside the partly broken door to the room where he’s trapped that leads from the upstairs of the building next door. If you break down the door in melee then you’ll get knocked back and prone, so make sure you have a party member standing out of the blast zone so they can get into the room without wasting time getting up again.
Oh, and you don’t need to rescue Benryn in order to rescue the Councillor, who is behind the door which the Flaming Fists will lead you to.
I don’t think I’ve ever had the problem you mention with pathing. If you have a savegame from which the bug is replicable then it might be worth sharing with Larian … unless someone else confirms they’ve already done so.
