Would it be plausible to say that the Soviet Union could have rolled past all of Germany and go on to liberate France and Italy before the British Empire, US, and Free French liberated these countries?
No. In fact, obviously not. Stupendously obviously not.
Because for Soviet forces to reach France, they have to pass through Germany. That means the German army and Nazi state are destroyed first. And it would
all be destroyed.
The Germans under Hitler did many stupid things, but they were not stupid enough to leave a huge occupying army in France while Soviet forces are storming Berlin.
Suppose Soviet forces reached eastern Germany before western Allied forces even landed in France. The Germans would strip France of German garrisons to reinforce the defenses of the homeland, leaving only the absolute minimum behind.
Unless the western Allied forces were completely paralyzed and cowardly, they would invade France which would be defenseless. Even if Britain and the U.S. were paralyzed, there would come a point where the French Resistance could defeat the remaining Germans.
The Germans might even withdraw all their forces in much of France, leaving it completely open to the Resistance and western Allied forces. This is what happened in Greece in 1944.
Beyond that: suppose Berlin, Vienna, Munich, and Hamburg have fallen to the Soviet army. What are the German troops in France going to do? Answer: surrender to the first Allied forces other than Soviet to show up.
This covers France and the Low Countries. In theory, Soviet forces could bypass Germany to invade Italy. But it would make no sense for Soviet forces to batter their way across multiple mountain ranges to get to Italy while ignoring the far more dangerous enemy directly to the wide-open west.
On top of this, the western Allies were in far better position to attack Italy than the Soviets. The western Allies liberated half of Italy while the Soviets were fighting to clear the Germans from Russia and Ukraine.
A slightly more plausible scenario could follow if the U.S. did not enter the Hitler War. The Soviets do crush Germany, but Britain, by itself, hasn't been able to muster a serious invasion force. When Germany collapses, the French Resistance seizes control; and a large element of the Resistance was Communist affiliated (the
Partisans et Francs Tireurs).
The Soviets racing across Germany enter France and insure that their Red allies take power, somehow preempting the nearby British. Something similar happens in Italy.
Even this is improbable, because it assumes extreme weakness and paralysis of Britain.