When you get the normal objective that says “No Firearms”, the dart guns will cause it to fail. It’s only the Prestige Objective that is tied to SA where they are fine to use for some reason.
Last mission of the last syndicate. Killed the wrong target. Did smoke, did drink, had all the visible clues. Forget to check if she actually attended a secret meeting. Failed and lost the whole progress. Have to start from the beginning. It’s my own damn fault but it still hurts.
All disguises can be used only once to reset the timer as it says there, it would have been even more impossible (had I not just gotten lucky and blasted the right target, but in hardcore mode you can’t afford getting shot at).
You keep the guns when you leave the level early. You can even gain a new one as long as it’s in your pocket when you exit the level. As others have pointed out you must actually leave the mission and go back to the safe house. Using pause and exit option counts the same as if you got shot to pieces.
If you play SASO and kill target with poison you don’t care about NBF option because is an accident kill. It’s implicit that the NBF option is connected to a NON accident kill.