Not so sure about that. The Soviets were very dependent upon both USA and our help to keep going. I don't think their mechanisation would have struggled without US trucks and the supply routes were pretty important.
en.wikipedia.org
en.wikipedia.org
en.wikipedia.org
I'm not so sure about that, odds are that the UK convoys and slightly later US supplies kept them motivated to stay in the war early on, but once attacked and pushed to total war I'm inclined to believe the Soviets wouldn't have backed down, they'd lost a lot of territory and the political implications were severe. They would have taken a good deal longer on their counter offensive without the support, but it would have happened and been just as devastating.
The one thing that WW1 and WW2 proved comprehensively is that the Germans can NOT win a protracted war where attrition is a major factor, they simply didn't have the raw materials, supply lines and resources, even if they had taken the Soviet oil fields I doubt they could have held them. The Soviets weren't efficient, but they had such a manpower pool and industry that they would eventually have taken them back.
Britain while under severe pressure from the U boats had a gigantic merchant fleet and the Empire/Dominions to call on and while the RN was pushed it was still big enough to keep things together on the supply chain front.
As to successfully invading Britain (if a peace couldn't be forced, which with the likes of Churchill I think is unlikely to have ever happened) I think that's unrealistic. The RN and RAF were big enough to maul any channel crossing and by the time that the Germans could have attempted it, Dunkirk was long over, Britain would have been re-armed and even if there was a beachhead I doubt it would have held, the RN could afford to lose a lot of ships opposing a landing and chewing up the German supply lines to the point of choking them to death. There have been a few threads on that subject.
To answer the question:- No I don't think they could have won.