While you make some interesting points, your example of the plane hijacking is irrelevant and just a failed debating trick.
We now know that a plane hijacking is an immediate lethal threat. Implying that the wait and see attitude is related to attacking someone with a known plastic gun makes no sense.
Also, engaging in physical combat with said plastic gun carrier makes no sense. I'm glad that folks have such confidence in their physical ability but once in close contact, your risk level goes up dramatically. Said plastic gun person might easily have a knife and deploy such on you.
The interesting question is whether if you are sure that the gun is fake, you kill or attempt to kill the person under the cover of:
1. The law doesn't distinguish between fakes and real (because it assume you don't know at the time).
2. You think you can get away with it and thus take the opportunity to kill someone as that is congruent with your personal 'philosophy' , personality structure or fantasies of monetary/societal reward.
Taking a life for those reasons is rather problematic. The use of lethal force is usually to prevent grievous bodily harm. Thus, you know that the weapon is fake and will not produce grievous bodily harm and you decide that to be executioner. Does the law in your state mandate capital punishment for an armed bankrobber where no one is killed? However, you decide given your perfect knowledge of the gun being fake that you will apply such a penalty.
If you don't know the gun is fake and the situation is such that shooting will produce the best outcome in protecting others from grievous bodily harm then I have no problem with it.
If you know the gun is fake and you decide to use that fact to safely shoot the guy - that's not in my play book.