Thank you, Joel. Setting aside the nonsensical overestimation of Axis capabilities, Hitler was Europe's problem, and he was their own sins coming back to haunt them with a vengeance. You are pretty much posing a false question, because if we had truly had a policy of isolation, there would have been no Hitler in the first place. It was our weighing in on the side of Britain & France in WW1 that led to the defeat of Germany and excesses of Versailles & Triannon, which in the first place, gave the German electorate sufficient grievances to elect their own early model of Trump. The Final Solution was not implemented until after we were at war with Germany, probably because there was no need for him to be concerned about world opinion anymore. Japan conquering Asia (again, nonsense: Asia includes the two largest countries in the world, with formidable geographical obstacles, and Japan was a glorified third world country back then. They had some top-down impositions of technology, and a handful of comparable weapons, but the technological gap between the US & Japan was equivalent to the British versus the natives in their colonial wars. Japan was going to be, at worst, bogged down in China for generations to come. They couldn't stand up for five seconds to the USSR, the most deficient and inept military power of the first world. As far as China's fate, I see absolutely no improvement between the hypothetical horrors of a Japanese conquest and what actually happened.
Would that they HAD kept us from acting! They didn't prevent us from poking Japan with a sharp stick, they would not have bothered with it.
And if we didn't push our way into the war as a backdoor to saving England and France from the consequences of half a century of diplomatic & military incompetence, there would have been no blood whatsoever.
If only there was an ambitious and militaristic nation in East Asia to serve as a local check to their power...oh wait, there was, but we destroyed it.

What is the point of being a super power if you have to jump every time one of your lesser twitches? Isolationist superpower worked just fine for the British. It was when they ended their "splendid isolation" that they got suckered into power games against first Russia then Germany, and ended up depleted and exhausted and forced to pass the crown to the US. If we let China et al lure us into the same sort of pissing contest, who will be there to pick up our crown when we go the way of Britain? I don't see any good options there, either. The surest way to come under China's thumb is to stoop to her level. Always punch up, not down.