I didn’t say it would have been cheap or easy. The Russians had already moved factories into the Ural mountains and were prepared to fight all the way there. And Lend-Lease did help them and give them more options.
But they still did the lions work and the lions share of beating the Germans. If we had never invaded in the West, the Russians would have eventually won.
The only fly in that would have been Germany developing the atomic bomb. That would have changed every scenario.
Most unlikely.
First, Germans were consistently victorious before the U.S. invasion of North Africa forced Hitler to withdraw troops from Russia to bolster Rommel in Africa.
By 1944, fully one third of German forces were defending the western front, troops who could well have kept Hitler victorious in the East for years longer.
Second and more important, without the US/UK second front, Stalin would certainly do what he seriously considered anyway: made a separate peace with Hitler, thus locking in Hitler's gains and freeing his forces (as in the First World War) to face any threats the West might pose.