Funny, but Stalin wouldn't have agreed with you. He demanded that the western front be opened up and as Italy was essentially a stalemate the Normandy invasion was the only way that was going to happen. Obviously, he felt Normandy was decisive.
Could the Russians have won on their own? Maybe, maybe not. What is certain is that without the huge amount of equipment sent to them by America with Lend Lease and by the British as well they wouldn't have been very likely to have held on. True, they had vast reserves of manpower, but manpower without equipment is just so many targets as the Russians well proved when they sent men into battle without weapons and expected them to get a rifle from the dead.
Frankly, I'm sick of all this "they did it all" deal so far as the Russians go. They didn't do it all. The Allies did it together. Many battles were decisive. Midway was decisive, El Alamein was decisive, Guadalcanal was decisive and oh so many others.
In all actuality the two most decisive battles aren't even in your list. The battle of Poland was the most decisive for Germany because it ensured that the Allies would eventually oppose them and put an end to appeasement. Germany never had a chance to win against the combined might of the Allies. And yes, Russia did take half of Poland in that bargin, but that only ensured that Poland would no longer be a buffer between them and Germany and Hitler's dream of leibensraum would go on so that they too would oppose Hitler. Likewise, for Japan Pearl Harbor was the most decisive as it ensured that their dream of expansion in the Pacific would be put to an end as well.
"Do not forget your dogs of war, your big guns, which are the most-to-be respected arguments of the rights of kings."
- Frederick the Great, King of Prussia