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.
I'm having a lot of problems with the game selecting the wrong level of multi-level areas since I picked up this playthrough after a month or two off. It's a major pain and I don't know what's different that is causing it. I seem to have a bit more lag, as well, but that might just be my memory playing tricks on me. Either way, I try to click where I want my characters to go, and it'll frequently target something on the wrong level (usually under) and the characters will scramble around trying to find a path, usually running in the opposite direction of where I'm trying to go, and disappearing off my screen before I can figure out where they went to.
In an area full of timers and hidden triggers for scripted events, this has led me to endless reloads and quicksaves just trying to do simple things like 'Go up the stairs, cast create water, and kick that door down.' Or 'Break down this door and interact with that character to free them from the rubble.'
I spent an hour yesterday just trying to accomplish thirty seconds of gameplay, and gave up. I'm stuck at level 4 and want to get to level 5 before I confront the Hag, so I can unlock some new spells, but I'm about ready to shelve this game entirely until August 31. It's so frustrating.