Bob, I grew up with the same widespread opinion. The British on their own were capable in holding the Germans, but not to defeat them. In combination with the US (or the US on its own) they could have done that without the USSR. Both the Soviet and Western blocks were capable, each in their own right, of doing that. In combination they were unstoppable and together shortened the war.
The only way the Germans could have won would have been a failure of will on the Soviet's part. The success of
Barbarossa would have been the only cause of that. I feel it is important to not only evaluate simple possible combat strength but tempo of operations in gauging relative strengths. If a side can keep its opponent always on the back foot, it has a very significant advantage that can be used at what it regards as critical points. In physics momentum = mass * velocity and can be used as a parable for the '41 USSR situation. The Soviets had the greater mass, but the Germans had a great velocity advantage and used that to smash the Soviets. However once the velocity dropped (resistance, logistics, exhaustion, etc), the German advantage disappeared. Sheer tempo is a weapon in its own right, something that I feel is too often under appreciated.
While Japanese intervention in the Soviet Far East might have made the German position a bit better, I seriously doubt that it would have changed the outcome. In such a situation the Soviets would not have been able to repeat Khalkhin Gol for some time, but the Japanese would not have had the strength to seriously weaken the Soviets. The Soviets did take units from Siberia. However Siberia as an entity is bigger than the whole US. Where the Soviets fought the Japanese in '38-'39 was regarded by the Soviets as the Soviet Far East. So a 'Siberian' division could be recruited many hundreds of miles from the nearest Japanese. The Soviets kept from 1 to 1.5 million troops in the Soviet Far East 'just in case'. While they did syphon off better units for use in the West, they seemed to do so only after building fresh units which would be sufficient.
There was some previous discussion on the matter and rather than repeating, I will refer to my part at
http://www.gamesquad.com/forums/index.php?threads/t-34-the-best-tank-of-the-war.118625/page-6#post-1697711.