I found a small bug where Nicolas respawns after being pushed into the molten steel pool.
To get this bug to happen, I first got the mission to Assassinate Nicolas from Olivia. Then I went to the metalworks and pushed Nicolas into the molten steel. I went back to Olivia, got the reward, and then killed Olivia with the mine cart. When I next went back to the metalworks map, Nicolas had respawned, as well as Agent Steve being present and mentioning Nicolas's death.
I haven't seen any consequences to this, just thought I'd mention it. I'm guessing there is a respawn boolean sentinel set to true after I killed Olivia, after already killing Nicolas and turning in the mission to Olivia. Just a guess.
Edit: I haven't been able to talk to Nicolas. I think the game is coded to expect Nicolas to be alive after you kill Olivia, even if you kill Olivia after killing Nicolas. I wonder if I kill Nicolas again (somehow), will Olivia respawn?
Edit 2: I killed Nicolas (again) in regular combat, ran to the next zone and Olivia did
not respawn. So, probably a flag set on quest return to Olivia makes Nicolas reanimate?