1.) No, it's a trait a weapon has. The blowgun has the non-lethal trait but does piercing damage.
2.) Yes from what I can tell.
3.) Unsure, I'm not familiar enough with PF1 to say.
It looks like they just deal damage normally but if you reduce someone to 0 they are knocked out automatically instead of killed. To do the same thing with a weapon without non-lethal you take a -2 to hit (-2 to hit for the blowgun if you're trying to do lethal with it.)