“what if Hitler hadn’t attacked Russia, at all, and had instead saved that energy for attacking Great Britain”
Wouldn’t matter if he couldn’t muster a ground invasion against Britain, and he couldn’t do that without defeating the British navy, which he failed to do.
Not going into Russia would have provided Germany with time and resources to confront the Royal Navy and finish the RAF. After that, Germany would have had 100K extra men to throw at GB. Meanwhile, the V2 would have been perfected and pounded GB. The RAF would not have been able to penetrate to Germany to attack the war factories US entry into the war could have been stalled by the extra attention on GB. (By the way, this is all speculation.100K
Add to that the finality of the air war over England. Without control of the air, Hitler’s dreams of invading England were finished.