Well, a few things. The Japanese atom bomb thing is bunk, they had no capacity for an industrial undertaking of that magnitude, although I know the story you are reffering to. It belongs in an Indiana Jones movie.
...Secondly, it was also the industrial capacity of the Soviet Union, which was immense, coupled with the USA, Britain and Canada combined, that did in the axis.
Germany had a slim chance of victory in Europe, if they had been able to defeat the Soviets quickly (or on the other hand, not invade at all), whereas Japan had really no hope in a war of attrition vs the USA, in any scenario.
If Germany did not invade the Soviet Union, they could easily have swept through N.Africa in 1942, there would have been no "two front war" with the Allies. What do you think they would do with all those unused Army Groups, send them home on holiday? Without an invasion of Russia, Germany could have consolidated its gains in Western Europe, and the Med, its invasion of Russia was its undoing from the first weeks. It was a fatal mistake.
As for Sealion...I agree with most of the opinions expressed, save you need to take into account the unprepared nature of the British Army in 1940 in the UK. It had few heavy units, and was hampered by a lack of transport, and anti-tank guns. It was nowhere near as formidable a force as the German Army in France in 1944, and any comparison is erroneous. The Germans on the other hand, were flush with heavy equipment, artillery, AFVs, and a sizable air transportable/para force.