🍁Nine Ancient Obelisks

Well, they were already leveling up during the game.

And... what is the "killability" parameter of a town governor like Ginza?

There is no such parameter. If we assume that he doesn't have a lot of HP and no defenses (although, I'm not saying that's the case), then he could be killed with Inferno just as easily as Ushumgall hit the bartender.

So, I'm asking: if you dive from the shore of Asuguh and surface on the shore of Calcite, bypassing the wall, how long will it take?

I'm not counting hours and days right now; if you want, you can try it yourself, as underwater breathing allows.