We could go round in circles on this point, but personally I believe that the Germans lost the Eastern front because Mussolini screwed up the Greek campaign.
Hitler had to delay the invasion of Russia by a few months while he closed up the hole in his flank. Then he had to weigh up attacking at the wrong tie of year, or waiting until the following year (and facing a better prepared Russia).
As for this Churchill thing, it depends on when that car crash was. He didn’t just pop put of a box to replace Chamberlain, he was doing other things. What knock on effect would his absence fro other events have had?