Hitler not declaring war on the US has to me always been absurd. Hitler always saw the United States as a "bastion of Judaism," and would have attacked it sooner or later. And even if he doesn't declare war right away, the United States will. Popular opinion was against all the Axis nations, not just Japan. Remember, this was still pretty soon after the Ruben James incident, and that's easily a cause for war if Roosevelt chooses to ask for war.
As for the capture of Moscow, it's only critical to the Soviets in the first year of war. If it falls in 1941, there really aren't any other rail links from north to south on the West bank of the Volga, and as a prestige thing, it's huge, because the Soviet Army hasn't been able to stop the Germans short of their capital city. Even Soviet production wouldn't help if the population running the factories doesn't have the will to fight, or deserts the machinery.
Moscow falling in 1942 or later will matter, but it won't be absolutely critical to the Soviets, as by this time, there was another rail line running east of Moscow, but west of the Volga. Also, the Soviet Army will have stopped Germany short of Moscow once, they're still fighting in Leningrad, and if Germany decides to make a run for Moscow, they won't be doing an attack towards Stalingrad in 1942. Soviet production and Lend-Lease will bury the Germans in an avalanche of production eventually, even if it's not in 1945.