Of course the only reason that Russia was fighting Germany is that Hitler double crossed Stalin after they had divided Poland between them. The USSR would have remained Germany’s ally except for that.
If Hitler had abandoned Barbarossa at an early date I think Stalin would have been satisfied with protecting his own borders and this would have freed Hitler to move the bulk of his divisions west. It is very unlikely that we could have successfully invaded mainland Europe if he had done that.
But I don’t think Hitler had a chance of winning WWII. The British Empire and the USA would have continued to control the seas and to choke off Germany’s ability to trade. We likely would have had a stalemate in Europe until the atomic bomb made its appearance, or until the German officer corp finally managed to succeed with one of its attempts to assassinate Hitler.
I'd guess that they'd be pretty happy about that outcome, and would have no reason to off the fuehrer.
Plus with mainland Europe under his control, Hitler would have almost limitless resources to build his OWN A-bomb.
At such point, we couldn't touch him.
If we threatened to take him out with our nukes, he'd simply say, ok, if you TRY, I'll nuke the UK.
So under such a situation, I'd wager that stalemate would have been the outcome.
Mainland Europe would be Germany, and the UK would be "allowed" to live, so long as we kept to ourselves.
I imagine the scenario would be a cold war of the US vs. Hitler's Europe, instead of us vs. the Soviets.
Now, about Hitler's health, that's another scenario.
He probably wouldn't have lived that much longer after 1945.
Probably dying in the '50s.
I don't know who would have taken over the reins at that point.