Yeah, there's a lot of factors and variables to calculate. We can never know what could have happened, only speculate on it. Personally I think that Germany could never capitulate the UK nor the US. Maybe they could try something on the UK if they did develop nukes earlier than the yankees and capitulated the USSR...

But even then, knowing very well what nazis do to communists and Slavs, the USSR would fight to the last person to win. I don't think Germany would ever be a par with USSR's manpower, industry and military genius (of course they got beaten hard in the first two years of war, but they managed to comeback). I think even on a 1v1 Germany would eventually lose. Idk.

When the US joined the Soviet Union was already fighting. Britain wasn't on the verge of defeat at all. They're being bombed, sure, but they were under no risk of a naval invasion. The USSR on the other hand was getting brutally attacked in its own soil.

In 4 years of war (for the USSR) they have lost about 25 million lives. 6,25 million deaths per year.

You're just watching too many movies. Every single person that made the same remarks in real live didn't do very well on their plans.

If NATO sends troops to Ukraine then Russia will really escalate the war and things won't go well to either side.

I think the US would join the war even if Germany didn't declare on them. But it sure made the US population more keen to fight when war was declared upon them than the contrary.