Same with Alderaan - they wouldn't really know what the Death Star was, since Tarkin never made contact nor responded to any

That isn't true. It's obvious that Bail Organa and probably his government knew what the Death Star was, after all it was his daughter who'd just risked her life to steal the plans to it, and he is one of the founding members of the Rebel Alliance. That much is fact presented in the other movies, without needing to delve into extra material. The Death Star wasn't a complete unknown, the Rebel Alliance knew about it. And seeing as Bail Organa and Alderaan's Rebel members knew what it was, they very well would have tried to get people away. This is hardly making stuff up. And besides, you're missing my point anyway, which is that I'm extending the destruction of Alderaan and so there is most definitely time for people to cry out in terror. But even without this, I don't agree with your suggestion that nobody knew what was coming.

And throwing that back to the catastrophes in Japan, they did have air raid sirens. People might not have known the scale of the devastation about to be wrought on them, but they knew something was coming. And again, catastrophic as those two bombs were, they didn't instantly vaporise everything. These things take a little bit of time to spread, and in that time, there is panic and terror. Obviously the blast is incredibly fast, but you make it sound like it instantly destroys everything in a millisecond, which is just not true. Do you really think one laser beam from the Death Star, however powerful, would cause an entire planet to go kaboom in a split-second? Yes, I know, this is sci-fantasy, and "anything" is possible, but I don't buy an Earth-like planet exploding at that speed. Anyway, you're obviously not going to agree, so let's leave it there.