and everyone else allows them to do so
Reasonably plausible. We can see from these excerpts that the United States had, even then, a deep distrust of the Soviet Union, and that as late as September 1938 Britain and France were willing to do almost anything to avoid military conflict with Germany.
"The Soviet occupation of eastern Poland in September and the “Winter War” against Finland in December led President Franklin Roosevelt to condemn the Soviet Union publicly as a “dictatorship as absolute as any other dictatorship in the world,” and to impose a “moral embargo” on the export of certain products to the Soviets." - U.S.-Soviet Alliance, 1941–1945. Office of the Historian, U.S. Department of State.
"As Hitler continued to make inflammatory speeches demanding that Germans in Czechoslovakia be reunited with their homeland, war seemed imminent. Neither France nor Britain felt prepared to defend Czechoslovakia, however, and both were anxious to avoid a military confrontation with Germany at almost any cost. " -Munich Agreement, Encyclopaedia Britannica
von Manstein, Guderian, and Rommel are drinking celebratory vodka shots in Moscow's Kremlin within half a year.
Unlikely. German intelligence heavily underestimated the reserves available to the Soviet Union. Barbarossa was based around an assumption that in addition to the roughly 150 divisions available to the Soviets, only 50 divisions of reserves could be mobilized. In reality several hundred divisions worth of reserves were available to the Soviets.
"German intelligence failures played a large part [in the failure of Operation Barbarossa] on several levels. The Red Army had been viewed with distain, especially because Stalin’s purges of the late 1930s had removed thousands of its officers - albeit temporarily in most cases. Its poor performance against the Finns in the winter of 1939-1940 also encouraged the Germans. Soviet industry was deemed incapable of producing modern weapons. Most importantly, Russian troop numbers and fighting strength were continually underestimated, so that despite the losses inflicted in early encirclement battles, the Germans always faced yet more reinforcements. The High Command had only considered the Soviet western army groups in their planning, and the presence of reserve forces and uncommitted formations in the Russian interior or on the eastern borders were disregarded. Even after Operation 'Typhoon' ground to a halt in early December, the Germans still chose to believe that the Soviets had nothing left to stage a counterattack." -Senior Curator Ian Carter, "Operation 'Barbarossa' And Germany's Failure In The Soviet Union", writing for the Imperial War Museum
This erroneous assumption of a weaker Soviet Union was a major variable in the equation of Barbarossa. The goal of establishing the A-A Line and the dilution of German strength on the Eastern front can in some part be attributed to this faulty intelligence. The German High Command planned Barbarossa as a short-term campaign and assumed Soviet resistance would always be below par. This assumption allowed for generous distribution of troops along the Eastern front. Without significant troop concentration and facing the prospect of an 1800 mile front, the Wehrmacht was fighting an uphill battle from day one.
The German leadership continued to underestimate their foe in the East and place far too much confidence in their own capabilities well into 1942.
"There were daily quarrels all summer. The point upon which we had our final disagreement was the decision of an offensive on the Caucasus and Stalingrad - a mistake, and Hitler didn't want to see it. I told him the Russians would put in another million men in 1942 and get another million in 1943. Hitler told me that I was an idiot - that the Russians were practically dead already. When I told Hitler about Russian armament potentials, especially for tank materials, Hitler flew into a rage of fury and threatened me with his fists. " - Franz Halder, "The Nuremberg Interviews"
Of course other variables such as the weather, logistics, oil production, morale, and treatment of the local population by the occupant Wehrmacht should all be taken into account when forming an opinion on whether or not the German armed forces could have won in the East. In my opinion however, any tactical advantages gained by favorable adjustment of those additional variables is dashed by the simplest of mistakes; the Germans, like much of the rest of the world during that time, underestimated their opponent.