There is zero evidence that Zimmerman had any status as an officer of any kind, either private or public.
His being a volunteer with the Neighborhood Watch confers no elevated status of any kind.
The media's choice to keep repeating the word 'Captain' as though it were an awarded rank seems overblown. The NW has no ranks, and the position of 'block captain' is a record keeping point of contact, not a leadership position.
So taking away any official status, and specifying private citizen to private citizen, the law doesn't spell out every possible twist and turn that could conceivably occur, it uses parameters like 'reasonableness', and tests developed over years of cases and rulings.
So the general answer to your questions about pre-emptive force is 'It depends'. In once set of circumstances, 'And then I saw the gun' might be deemed reasonable. In another set of circumstances, it might not.
And that is without factoring in self serving accounts by the actors, witness unreliability, prosecutorial misconduct, inadequate representation, jury bias, jury nullification, and so on.