1) Hitler's true goal was Russia in the first place. He simply doesn't want the west to interfere, hence taking them out first.

If the West didn't react then Hitler would have attacked the West nonetheless. Poland was the bufferzone vs. Russia. The idea is not to alarm the USSR, hence the non-aggression pact ( I can't exactly remember when this was signed).

The other three are too extreme to consider. The world was weary of the war. They simply wanted it to end. Hence the atomic bombs, thus Russia wouldn't attack Japan themselves and complicate things up.