So, for a single fight, that was about two hours of "gameplay" with at least 6+ reloads just to learn script triggers, gotcha enemies, and unpredictable NPCs, and I barely feel like I've made any progress or had any fun for my gaming time. You're really better off going into this area hard and just killing everyone, but then you won't get to experience the developers' awesome "puzzle".
I'd say some of these things you describe are bugs, but they never happened to me. First off you don't need a barrel to blow up the cave-in, but nice that you solved it this way.
After I cleared the cave-in it became clear I'll have to kill Nere, but I thought I'd kill off the dwarves first. I positioned Gale and Shadoheart on the high platform above the cave-in area. That's the platform you can move up-down and forward-back with two levers. My PC and Laezel remained down. I cleared the dwarves in like 6-7 combat rounds. Then Nere was without buddies when I freed him.
And there are multiple other ways to go through freeing and killing Nere, including winning some of the dwarves over on your side. Look it up back in the thread.
When the cutscene placed me in a fight in an open area surrounded by enemies except for the lava pits, I did reload, sent all my characters up a nearby ladder onto a stone walkway, and threw bombs from there to open the passage to Nere. The same cutscene triggered as though my PC were standing down below holding a conversation with Nere, but the entire party still appeared up on the stone walkway at the start of combat, which permitted me to kill the enemies as they arrived, beginning with Nere himself. I also used ranged attacks to kill two irritating spellcasters who remained below, and finally sent my paladin and barbarian down to elminate the three enemies left (two ranges and one melee who, for whatever reason, never ascended the ladder).
Using cutscene magic against the enemies.