You forget that this was the time before effective delivery vehicles were available - all you have is bombers. The reason why it was so easy to nuke Japan is because their air force was already decimated. I don't think that a bomber carrying a nuke to, say, Berlin would have a good chance of getting there if you take the hypothetical "Great German Reich from Atlantic to the Urals" scenario, with all the industrial base and resources of Europe and USSR at its disposal. You'd need to send several to get any reasonable chance of getting one through, which is very costly, and even then would probably only work for the first attack, since afterwards they'd just start going after bombers disregarding any losses.
No, the nukes were as much a wunderwaffe toy as German V-2. What won the war was the determination and courage of soldiers on the ground, and the sheer manufacturing capacity that Allies had - US and USSR both. The turning point was really in 1941, when blitzkrieg in Russia failed before taking over Moscow, and Soviets succeeded in fully relocating their industry to the Urals and getting it up and running on full capacity. Germans were good at blitzkrieg, but they simply didn't have the resources for a prolonged meat grinder kind of warfare, and that's what they got into in the USSR.
As far as who contributed most to victory, I think it's a fair assessment that USSR bore the brunt of the war in Europe, and US did the same in the Pacific. Everyone else was kinda tagging along - yes, even Brits. It's all really obvious when you look at losses and inflicted casualties. 5 out of 6 Axis soldiers killed in Europe died on the Eastern front, and this also represents about 2/3 of total Axis casualties everywhere. Most of the remaining 1/3 are Japanese soldiers dead from American action.
As far as lend lease goes, it was certainly a noticeable aid to the USSR, esp, in 1941-42, but it is unlikely to have been decisive - it didn't play much role in stopping the Germans at Moscow in 1941. Without it, it would probably have taken an extra year or so for Soviets to build up enough to be ready to go on the offensive, so Kursk (or equivalent) would have been somewhere in mid-to-late 1944, with the corresponding cost increase in human lives. So I think that USSR is by all rights entitled to a claim of being the country that defeated Germany in WW2 even with lend lease in the picture, much as US can claim victory over Japan.