Well thats always seen as Hitler's greatest blunder - not finishing off Britain before invading Russia. Hitler assumed too much - he assumed he would have moscow by winter - he assumed Russia would fall easily. Of course, as we have found out, that wasn't true and guess who ended up kicking his ass in the end anyways
I doubt the U.S. would've been involved earlier Nimbletoe. The U.S. saw no reason to be involved in Europe, and it was really only Pearl Harbor that completely changed public opinion to go 150% for the war effort (not to mention Hitler was the one who declared war on the U.S., not the other way around, and his declaration was really of no reason and only gave FDR the war he wanted).
As for Stalin... I don't know whether or not he would have attacked Germany. Stalin was HARDLY in a position to invade the best military in the world at the time.
A few reasons:
a) Stalin believed that Hitler would keep the non-aggression pact. While its hard to predict what Stalin would do, files found in the recent years have suggested that Stalin wasn't thinking war would occur.
b) The military and economy was hardly ready for war. The military had 3/5 of its officer core 'purged' by Stalin so it was severly lacking.
Stalin was NOT in a position for a war - hence in the first year of the war, he was nearly defeated by Germany. You don't lose 1 million troops in the first month or two of fighting if yo uare prepared - much less do you invade if you are in such a bad position.