The US were not remotely involved during the Battle of Britain. The US in fact refused to get involved as they thought the British were going to lose and it was the Battle of Britain and the British winning that caused Hitler to turn his attention to the Soviets.
As for the "had" argument, well you are doing that too, only you are saying "had the Americans not been involved then the Germans would have won"
Had the British sued for peace in 1940 before the Battle of Britain Germany would have had an extra 750,000 troops for Operation Barbarossa (250,000 that were in Norway/France to ward off British attacks, and 500,000 that were in North Africa), Germany would have also had an additional 2000 planes and some of their best pilots that were lost in the Battle of Britain and would have been able to attack the Soviets in May 1941 instead of June 1941 (since the British involvement in Greece forced a delay on the invasion of the Soviet Union).
So without the British, Germany would have had 2000 additional aircraft, 750,000 additional troops and an extra 4 to 6 weeks to invade before the Russian winter hit.
It is so easy to play the "had" game, and I don't think you should pretend that you are not playing it as well.