The further south that the waterways get captured, the less is the period on which they are passable on Ice. Hence my point about the key to conquering Russia being a southern front from the Black sea, - invasion followed by an advance on the nothern front. Had I been Hitler, that's how I would have tried the initial plan. And he did, but the British were in the way, and even the Spanish. Then he did the biggest mistake of all, which was the "southern swing" instead of focusing on Moscow.
So perhaps the turning point of the war was when Hitler ordered Guderian to take his Panzer army on a few thousand miles trip before going off for Moscow.....