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Your notion of initiative -- or maybe it's permission -- is what's silly a bit precious. We're not talking about what's right or wrong, fair and honorable, expected vs unexpected -- even though these concepts do be at play.
However, a sucker punch is a SIMPLIFIED concept, and which isn't solely defined by whether the person expects to be punched. (Correct me if I'm wrong in assuming the bold part is what you're saying.)
Here's what we both might agree is a justified[1] sucker[2] punch:
You might argue that all the bullying leading up to actual sucker punch @:30 also qualifies, but you can see there's a difference between the two.
The 'sucker' in sucker punch, comes from the guy not KNOWING it was coming. He got suckered. He did not see it coming.
'HAHA sucker, bet you did not see that coming, did you'?
Its an extension of the old prior gun fight code, that has flowed into a fist fight.
Could you just pull out your gun and shoot someone in the old west, who you thought might be a threat, and might kill you if you fought him fairly, in a draw down situation. Sure. It might even be smart as you might know he is the far better gunslinger and will kill you in a 'fair fight'.
But it is still a sucker shot, to draw down and shoot him before he realizes it is a gun fight. This answer is NOT tied to who was right or wrong in whatever the dispute was.
In a fair gun fight, you don't just shoot first over the perceived offense in the bar. You tell the guy to step outside giving fair warning.
And we can agree to disagree. You can say 'no, you shoot first at any perceived slight, if you think you might die if you don't'. He spills your drink or jostles you and you think this might end up in shots fired, just pull your gun and shoot first.
Some guys does not even know he stepped on your toe or jostled your drink, just punch him in the face first if he dares to square up when you confront him.
If that is your view of the world and things we can agree to disagree as that is always a sucker punch to me.