Thank you for your well written answer and well thought out, Quid.
When I said that free will is one way to avoid the question, its because I find it somewhat problematic to say that free will coexists with a God who is omnibenevolent and omniscient. The problem arises when we see that people with agency have bad thoughts and actions, they conduct and execute their evil thoughts and actions against other people, hurting, torturing and killing because of this free agency that makes many people say that "God must be silent", that "He is totally indifferent" or even that "He doesn't exist". Considering this case of the genocide of Jews and other peoples in the Holocaust and all the tragedies you mentioned in your post, some people say: "Okay, because these individuals are free agents, an omniscient (who saw all this evil happen before the creation) and omnibenivolent God could do nothing about it? Would the will of these people who suffered and begged not to suffer and die be affected because of God's action?", "some people's bad thoughts and behavior were not avoided because of your free will and agency, but why the terrible consequences were not stopped by an omnibenivolent God?", "the idea of free will cannot adequately lead to the conclusion that an omnibenivolent God must endure their evil acts" or "in the Bible era God saved Israel people from Egypt, opened the Red Sea for them to cross, saved Israel from the hand of various enemies regardless of their free will, but only that it did not deliver its chosen people from a brutal slaughter because your free will?". These were some of the arguments I've been hearing over time And they too may raise other arguments similar to those of epicurus Ad Infinitum...
No, I never read Dostoevsky's works.
Okay, let us think for a moment: If the Nazis hadn't taken over, who would rule Germany? In 1918 as the house of Hohenzollern crumbled, Communists took over vast swathes of Germany, such as the entirety of Bavaria. The Wehmahr Republic was an unstable affair, and if not the Nazis, the Commies are a good bet. Communist regimes in Russia and China killed far more people, and were far longer lasting. In Cambodhia they killed a larger percentage of the populace.
What if Germany and the Soviet Union stood together against the rest? WWII was a close run thing as is, but I am not sure a German-Russian Pact might not have won. So Communism expands, what Great Leaps Forward, or Holodomors, or Purges, it might not have done? How much less would our material civilisation not have been to boot?
As this hypothetical makes plain, we don't know the consequences of events. We think we can envision a better world, but if you pull a string at one end, it might unravel the cloth somewhere else. Essentially this is a paraphrase of Leibnitz again, but there really is no objective way of determining if it is even possible to have a better world than our own - while still maintaining our levels of agency. Even something that seems an obvious good, such as not allowing the Nazis to come to power, might in the long run have had far worse sequelae.
This is the danger of hypotheticals and supposition. We simply don't know, and thinking it could have been better somehow, is merely a complete guess - not even an educated one, really. So rather look at what we can all agree on - what currently Is, or what has been (though of course mediated by the prism of our intellectual frameworks, and the biases of historians). As I noted before, I can't watch my son toddling about, or feel the sunshine on my skin, and not conclude "that it is Good". Others have far worse lives surely, but certainly the balance is far more toward what we conceive as Good rather than ill. I don't have the ability to take the cosmic view, but my subjective view and reading of history, supports the view of Goodness lying below the surface. We name Wars, we seldom name Peace - in general people live their lives contentedly for the most part, though there might be pestilence and war and famine about. Evil is the exception rather than the rule - no matter if we see Naturalistic Materialists trying to excuse altruism and goodness on fallacies of motive; or those that anthropomorphise Nature red in tooth and claw, as if this is somehow evil.
The Nazis were bad, but that doesn't suddenly mean we all are, or that the world is therefore indifferent. The Allies also did terrible things, or The Vietnamese Communists were also bad, but better than the Nazis or the Khmer Rouge they respectively drove out. To assume a better world maintaining our levels of agency is a possibility, remains a shot in the dark.