Are you kidding me? This isn't a technical problem, this us an" I looked up the speedrun on YouTube" and tried it myself. Save this page for people with ACTUAL game breaking crashes and bugs. All I can say is WOW! Smh
There literally is an achievement and a special reward(Dallis' Hammer) for beating Dallis and Alexandar in front of the ghetto and using Deathfog is simply the easiest and most straightforward way to do it. All the other ways are about as cheesy but take a whole lot more time.
Not to mention that it's probably an extremely common method as well as sort of poetic justice if you are playing as Ifan and/or Sebille.
The game glitches out a bit and does things it clearly isn't supposed to do, such Alexandar being in the Harbor but only as a corpse as well as the Voidwoken Drillworm spawning, ignoring the magisters in the harbour and going straight for you as well as a missed conversation with Alexandar. That is a problem, takes away fun from the game and should be fixed, I don't understand why this is such an issue for you.