I had (have) the same problem as everyone else, however I found a way to use the bug to my advantage, here’s what I did..
After activating the quest I bring everone down to the ambush spot but have Trahearne stop just before it so he doesn’t spawn them.
I then advance to the Abomination, then pull them all back to the group who then aides in killing them all without spawning the ambush (not sure if this was another bug).
I then solo my way through the tunnels and clear it out.
Go back to Trahearne and spawn the ambush and watch your lvl 80 heros get pawned lol, pick off the mobs 1 by 1, then finish off the quest.
Hope this helps as I spent over 12s in repairs before getting this to work.
