Basically no to all of the above. One can question the wisdom of not standing up to aggressive nation states attacking and conquering other states - and doing nothing. Let's say the US had invented the bomb earlier and was using this strategy during the rise of Nazi Germany. Basically the plan would be to leave Japan, Russia, Germany, Italy unchecked as they conquered state after state until the whole rest of the world fell under horrifying authoritarian rule. Until one eventually comes for the US and both commit suicide. It seems a much worse scenario than the already horrifying one we had.

On the other hand let's say all had the bomb but are reluctant to use it. The nation's fight it out with conventional weaponry until the axis powers, realizing they have lost and with invasion in progress finally feel they have no other choice and commit mutually assured destruction. I think it would be hard to claim that wouldn't have happened given the temperament of some of their leaders. Perhaps in this case the allies would have showed restraint and not invaded to end the war - perhaps they would have been willing to negotiate terms.

I think basically my idea is a terrible one. But I also think the world war II scenario would end in nuclear exchange - the only way to avoid that is not push any power to the point of last resort, and negotiate an unsatisfying agreement that leaves the aggressors in power, no matter what horrible things they do. Would all have shown such restraint, would cooler heads have prevailed amid the terrible atrocities of total war? I'm not particularly optimistic of the chances.