Again, you are not taking into account the stress Britain put on Germany's industrial military capability. Britain is an island and thus the Germans could never invade it, and the Royal Air Force starting in 1940 bombed Germany's heavy industry on the Rhine almost daily. This combined with the fact that the Wehrmacht was always bailing out the incompetent Italians in northern Africa, plus the heavy civil resistence in occupied Eastern Europe, put a great deal of strain on Germany and significantly aided the Soviets. This, combined with the fact that the Russians had 3 times more heavy industry, 2 times the population, 4 times the oil and knew the territory much better because they were fightring at home, made it almost impossible for Germany to win. And BTW, the Wehrmacht got to Moscow's suburbs and almost took it.
As for what you said about Hitler's options, he would have to take several nations before he could hit Russia from the south, and it's doubtful whether that would be somehow superior to hitting Germany from Eastern Europe. Germany's Waterloo was Stalingrad, and that's because Hitler insisted on taking the city instead of just wiping it from the face of the Earth. If I were Paulus, what I would have done would be to move my troops in two split groups, one to the south and the other to the north, then enveloping the city from all sides, barricating the city and stopping anyone from fleeing, and then just bomb the city to rubbles from the air and with long-distance artillary. That would have been the smart thing to do. Eventually, the few survivors would surrender and the battle would have been won with the Red Army troops locked inside the city unable to flee or fight back. But Hitler was a proud man who wanted to simply conquer the city in a direct assault, for no other reason than the city was named after his arch-nemesis, and he wanted to humiliate Stalin. Hitler's arrogance and obsession with man-to-man combat - to prove the superiority of German warriors - cost him the war. 
SUCKMYMUSCLE
You got this one wrong. After Dunkirk England was defenseless. Yes, over 300,000 Allied soldiers made it back to the island but they had to leave all heavy weapons behind. Of course had Hitler not blundered at Dunkirk but instead taken all the Allies as prisoners the invasion of England would have been as easy as 1-2-3. Anyhow, Hitler proceeded to bombard the British airfields and radar stations attempting to gain air superiority to begin the invasion. Had he not shifted his focus to bombing London out of rage he would have gained air superiority and German paratroopers could have taken all valuable airfields until reinforcements would have made it across the channel. In the same fashion the Germans took Crete from the British they could have taken a largely defenseless country/island in August of 1940. This would have kicked the British out of the war and all of Northern Africa, Gibraltar, Crete, Malta and the entire Middle east would have fallen into German hands.
Even with not invading England Hitler could have easily defeated the British in Northern Africa and the Middle East in 1940 right after the victory over France. This would have brought him to the gates of Southern Russia in 1940. Governments like Turkey, Iraq and Iran were all pro-German at the time.
Instead he chose to launch a full frontal attack on Russia in June of 1941. He underestimated the Russian military capacity and despite huge victories in the first few weeks he quickly ran behind his timetable. When he decided to shift his panzers from Army Group North and Army Group Center away from Leningrad and Moscow to take advantage of the Kiew encirclement, the war was lost because there was no way to make it to Moscow before the onset of the rain and mud season in autum. He should have been more daring and kept on moving towards Leningrad and Moscow while letting Army Group South take care of the Kiew encirclement by itself. The war was lost in 1941 by not defeating the Red Army.
It doesn't matter what happened in 1942. The Wehrmacht never regained its strength it had in 1941. Stalingrad or not, in 1942 Germany never again had the overall strength to possibly defeat the Red Army.
