The Sparkmaster Quest is entwined with the lighthouse guards quest and the Legion vault in Aureus' basement. Several possible outcomes, one of which is broken.
  • If you do the lighthouse first, and let the lighthouse guards be the heroes, one of them gets appointed as the new vault guard. You don't have to talk to him again to finish the quest - it happens when you all report back to Aureus.
  • If you let them dangle, they disappear from the game after visiting the lighthouse. Reporting to Aureus still finishes the quest.
  • I haven't tried setting them up and cutting them off at the knees with Aureus. Given what happens with the fish thief, that may give a third result.
  • If you save the Sparkmaster guys - either before you clear the lighthouse, or after telling the lighthouse guards to suck wind - then one of the Sparkmaster guys becomes the vault guard. Talking to him finishes the quest, and you have the same conversation bonus. Otherwise, they disappear from the game once they leave the mine, despite the quest description. In that case, you're SOL for finishing off the quest. This is either a design failure or a broken script. Dunno which.
  • If all the Sparkmaster guys die, I expect you can finish the quest by reporting to Cecil, but I haven't tested that either.
  • If you neither support the lighthouse guards nor save the Sparkmaster guys, the generic vault guard - who doesn't like you much - stays in place.
  • If you change the guard either way, you now have a friend on guard, and you get some new conversation options.

Another recent thread discussed how Perception of 11 or 12 will see pretty much the best-hidden traps; if I were a betting man I'd lay money that perception 9 is just not enough for the Witch's chest.