A: The portal was moved. The Black Morass is now the Swamp of Sorrows, I believe, judging from the terrain.

A: Dunno

A: If Hitler had been killed or stopped early, WW2 may not have happened. Let's look at one possible (not necessarily probably) consequence: Japan never invades East Asia since the West isn't otherwise preoccupied with the war in Europe. Since there's no war in Asia, there's no atom bomb dropped in Japan. Japan doesn't go through its horrific post-trauma stress disorder and its amazing nation rebuilding. Japan never (or takes much longer to) becomes the technological and innovation powerhouse it is today, and without the collective PTSD of Japan's artists, we never get blessed with anime and manga as we know it today.

I know it sounds like I'm arguing that Dragonball Z was worth slaughtering millions of people in the war, but I guess the point that the Bronze Dragonflight is trying to make is: better the horrific present that we know, than a potentially more horrific present that we don't.