I doubt if there is any way the US could have defeated Japan faster than we did, even had we ignored the European theater to focus all attention on Japan.
Remember, among other things: that effort required a huge Navy with over 100 aircraft carriers, large & small, and many hundreds more of other war and transport ships.
These were still being produced in 1945, and no way would the US have invaded Japan without them.
And in the end, the US did not invade Japan, instead our A-bombs finally convinced the Emperor the fight was over.
So it simply is not possible for the US to have defeated Japan before August 1945.
And had Germany still been in the fight then, the US would have turned our A-bombs on Germans -- since they are who the Bomb was originally developed for.
Also remember, history records several attempts by both Stalin and Hitler to reach a separate peace.
Thankfully none were successful.
In 1941 Hitler brushed aside Stalin's feelers, and after 1942, when Stalin was convinced of his ultimate victory, he would not entertain any German peace feelers.
But Stalin constantly demanded Britain and the US open up new fronts in Europe, to take some pressure off his armies.
And Churchill and Roosevelt constantly promised Stalin new fronts, beginning in 1942 (North Africa) and every year after-wards.
Had Stalin become convinced the US and Britain would not come to Russia's rescue, he doubtless would have done just what those same Communists did do in the First World War -- sign a separate peace.
Bottom line: all things considered, in a war where something like 75 million people died, the US achieved the greatest victory with the least loss of US lives that anyone could realistically imagine.
So any proposed alternate scenarios have a major burden of proof to show how they would achieve the same victory with even fewer US military casualties.
Unfortunately as beacame clear in a few years, it turned out to be a Pyrrhic victory with so many of the people liberated from the nazis only to fall under the dictatorship of the commies.
You assume there would have been combat with Nazi Germany in the short run. I disagree. Indeed, but for Roosevelt’s belligerence in word and action, and his attempts to lure hitler into giving him a casus belli in 1940 and 1941, Germany may well have not declared war on the United States oin December 11th, 1941, since the Tripartite Pact was defensive only.
As to the premise that we could not have defeated Japan earlier than 1945, several salient points. First, without war in Europe, the U.S could potentially have stripped, to some degree, her Atlantic fleet and sent it to the Pacific; much in the manner FDR stripped units from the Pacific Fleet to reiforce the Atlantic prior to Pearl Harbor. Second, without war in Europe, the U.S could have sent a far greater number of troops and supplies to the Pacific with the shipping they were using to convoy goods to Britain.
But thirdly, and most importantly, how were the Germans going to threaten us. First, a Navy that can’t reach England isn’t going to reach the U.S, especially with the Royal Navy in their way. Second, the Germans had NO strategic bombing capability capable of reaching the U.S at that time. Third, the Germans were already overextended militarily. The bulk of their Army was in Russia. They had troops fighting in Africa. They had garrisons in France, Belgium, the Netherlands, Denmark, Norway, Greece, Crete, Yugoslvia, Romania, you get the idea. They had neither the troops or air units to cause us any anxiety.
As to the atomic bomb, Germany’s development of nuclear weapons was derailed by-Adolf Hitler. He refused funding for scientific projects that couldn’t deliver the goods within a year or so. And Hitler considered physics a “Jewish” science. So that bomb was coming any time soon.
The U.S had strategic interior lines against its’ two enemies. They could have concentrated their forces, especially air forces, against the Japanese, crushed or left them to starve, and then turned their attention [if necessary] to the Germans. Japan didn’t have to be invaded. By 1944 we had destroyed almost all their fleet, left them with pilots suitable only for target practice, cut them off from food and resources, and could turn the entire country into a cinder.