As for the second part: presumably it wouldn’t change anything. People tend to sum up wars in one villain (it’s funny to see how in the world people think all our wars are Netanyahu). But the Holocaust wasn’t Hitler. It was Germany, and Europe, and philosophy, and politics, and even science (eugenics, for example). Hitler didn’t invent anything.
I’m not, God forbid, trying to downplay his importance; I’m just pointing out that he wasn’t the whole picture. If you had killed him, there would have been a Holocaust in 1949, Himmler would have been chancellor, and he wouldn’t have declared war on the U.S. after Pearl Harbor. Germany would have won, and you wouldn’t have been born to kill Hitler, which restores the order of things and so on in an endless loop.

Wow, what a sharply definite historical fatalism-determinism. I’m fairly convinced that’s not the case, but this is of course a hypothetical discussion with no way to decide it.

I was joking about the fatalism, of course—how would I know what Himmler would have done (if he even would have been part of the story without Hitler). It’s possible everything would have been different without Hitler. My point is that the Holocaust was much bigger than Hitler, contrary to how people usually see it.
Hitler joined the antisemitic Nazi Party in 55th place; the founder of the party was Anton Drexler. Antisemitism in Germany (and quite a bit of Europe as well) had reached a boiling point—religiously, economically, and culturally. Theories like “The Elders of Zion” were widespread, and in Germany there were dozens of popular antisemitic organizations and movements, with talk of ethnic cleansing or at least expulsion. As for the war, there was a sense of crisis and humiliation after the defeat in World War I, to which was added the economic crisis of 1929—which many openly blamed on the Jews. There were many figures with similar ideas.
Maybe Hitler’s rhetoric and stubbornness were necessary, but to me it sounds plausible that they weren’t. Presumably, Nazis under a different leader would have carried out a Holocaust on a different scale, perhaps even a substantially different one. But I wouldn’t count on any great salvation for the Jews.