You can make the steward immune to knockouts and immortal (just give him an enormous health pool). If you don't want the player to mess with the steward until he received the neccessary information you could leave the steward in a blue room and teleport him in his room once the it's time. A bit gamey, but should work.
Thanks Obs. Yeh I did that in the mission. It should all be solved now, all those problems.