Nazis defeat Stalin, check.
Nazis roll into Russian, begin the usual reign of terror.
Partisan warfare begins, quickly becomes brutal and vicious.
Japan attempts to capitalise on this by invading Western Soviet Union.
US tightens economic hold on Japan, builds up more forces in the Phillipines.
German forces redeployed to North Africa to assist there, British forces face a more difficult foe. However, this allows partisans in Russia to be more successful, so there may have been delays in the re-deplyments, or possibly units being sent back into Russia after getting to North Africa.
US naval units repeatedly encountering Kreigsmarine forces, direct action takes place. Eventually, somebody is going to get mad enough to declare war.
US forces in the Phillipines repeatedly encountering IJN forces at sea. If there is any American assistance to the Russian forces via a lend-lease program, there would be epected clashes with IJN units attempting to intercept said supply ships. Expect there to be direct hostile action. Eventually, somebody gets mad enough to declare war.
Roosevelt was very aware that any war with the Axis would have to happen soon, or the UK would be in a lot of trouble. There were several scenarios being floated where the US would be able to declare war on Japan (Like sending in a US Army manned sailing boat to spy on the IJN, and when it would get sunk, that would be a reason for a declaration of war. In reality, they managed to excape alive.
If the Japanese were foolish or arrogant enough to attack an US facility, it would be the justification to declare war, same for the germans. It was a matter of time before it happened, and both countries were arrogant enough to think they could get away with it (well, at least those in charge were).
I think a German victory in the East would definitely prolong the war though. It should have freed up a lot of troops to be used in North Africa, which would have been a serious problem for the UK forces there. Should the US be drawn into the war, it would have been most likely against the Germans alone, so that may have helped the allies in the long run in the North Africa/European scenario. Timing would be key.
Alvis 3.1