You overestimate both the power of Nazi Germany and any nation’s ability to launch a cross-ocean invasion without having a beachhead on which to amass troops. The D-Day invasion was one of the biggest efforts mankind has ever undertaken, and all they had to do was cross the English Channel into occupied territory where the residents wouldn’t fight back.
Anyway, the answer is that even given all the events in your scenario, Germany and Japan still lose. The U.S. was putting out immense amounts of arms at the end of the war, and hadn’t really strained its economy yet (income taxes were only at around 9% of GDP at the end of the war, close to what they are now). The U.S. economy grew throughout the war, whereas the GDP of all the other major belligerants shrank. Even so, the U.S. spent a smaller proportion of its GDP on the war than did anyone else, and had a lot of room to spend more.
To give you some idea of the economic power of the U.S., here are some facts:
The U.S. built more tanks in 1941 alone than the entire German production of tanks before and during the war. In total during the war, the U.S. built about 88,000 tanks versus 24,000 in Germany. But those numbers don’t tell the whole story, because German tank production was going full-blast in 1939 when the U.S. only made something like 300 tanks. If the war had lasted a couple more years the U.S. probably would have had 10 times the number of tanks as the entire axis.
At the end of the war, the U.S. was producing almost 20 times as many aircraft per year as they were at the start of the war. The U.S. built 303,000 aircraft from 1940 to 1945. At the start of the war the U.S. also had one of the least advanced airforces. At the end of the war, they were producing the best aircraft in the world, and they were producing almost 300 aircraft per DAY.
More importantly, the U.S. was digging in for a long war, and was committing billions of dollars to new factory construction. Germany didn’t have the resources to do that, because it had committed everything it had to its current inventory and plants. So the amount of manufacture would have continued to increase in the U.S., while staying flat in Germany for a long time after a cease-fire. By the time the Germans could ramp-up production the U.S. would vastly outnumber them.
The U.S. hadn’t come close to tapping its pool of young men to fight, and could mount massive armies. In contrast, Germany was drafting young teens and committing so much of its production to war that the people were in poverty.
Japan and Germany’s only hope in the war was that they take some ground, and make it too expensive to take back, and eventually sue for peace. Neither of them had a gnat’s chance of taking over the world, separately or combined. If Germany had managed to conquer Britain, she would have had no chance of negotiating a peace. The U.S. would have had to find an alternate route of invasion, or simply continue to develop high-tech weapons at a blindingly vast pace until they could launch ICBM’s or whatever. But Germany had zero chance of successfully invading North America.