No x, not under reasonable circumstances. I'm sorry, but the firing on a single fort in Charleston harbor simply does not give legitimacy to the military invasion and occupation of 11 states. Lincoln used it to do so, but not in any reasonable or legitimate manner.
There is a right of rebellion and a right to self-defense, but neither justified the assault on Sumter.
Sure they did. Charleston's right of self defense was being violated and threatened by the presence of a foreign military in its harbor. That military had no other business being there than to obstruct free entrance to that harbor and had already shown that was exactly what it was there to do a day earlier by firing on a confederate ship.
imagine if county or city police had fired on the state militia
Your analogy is false. The applicable description is of a foreign nation attempting to maintain a hostile army within the borders of its neighbor, as that is precisely what happened. Lincoln could not have expected to hold his forces there and exert them against entrants to the harbor without prompting action to remove them sooner or later.
What's striking is how some people who rightly object to excessive federal power and abuses justify the same sort of conduct when states or competing nations engage in it.
It certainly is striking, and stands out in perhaps no better example than those who dismiss Lincoln's conduct on the grounds that it supposedly "saved the union"
Perhaps you are correct, but remember, the established rule: the aggressor in a war is not the first who uses force, but the first who renders force necessary.