The OP as written is highly implausible (plus three posts one after another is bad form when two of them are bumps). On the other hand;
Handwaivem: Soviet Backstab on Hitler immediately after the fall of France.
Let's hand wave the how's, and just ask about the butterflies. Firstly the Red Army at this time, even with hasty preparation is going to perform horribly, and the Soviet offensive is going to stalemate fairly quickly in central Poland. The minor states of Europe, like Romania and Hungary are going to be backing Hitler and Germany against this Soviet aggression, meaning that the southern Ukrainian front may be terribly 'in flux' but the river line in Bessarabia might be a good boundary.
While the Soviets won't have their army shattered by leader apathy and German Blitzkrieg, they will suffer the problem of being 'the aggressive warmonger' as much as Hitler was.
Geopolitically I would suggest this would force Churchill and Britain away from the Soviets as much as Germany. While the enemy of my enemy is my friend, and Britain will be hoping that Stalin will kick Hitlers butt hard, he will also be petrified of a 'Red Europe' created by Stalin actually defeating Hitler.
This leds to the odd state of affairs where Hitler and Churchill might actually become the proverbial friends with a common enemy. It all depends on the deal struck.
For a start, France has been defeated on the map and spirit, but not economically and millitarally. Therefore the odd case might be for an Anglo-German armistice for Hitler returning France to french rule, and the French making a concession to Germany over historical territorial disputes.
If Hitler accepts such a 'deal' is unknown, but depending on Soviet successes, who knows...
In such a case it is unlikely Britain and France would be German 'allies' but a cessation of hostilities between the Axis and 'Entente' might be a fair trade. Hitler getting what he wanted; Poland and France and Britain getting what they want 'status quo'.
Of course France in general will be outraged, but also nationally crushed. This might lead to internal problems and a complete rethink of French geopolitical position (since it will have been treated by Germany and Britain like a second rate power).
With a 'Britain prepared for war', and a Germany rebuilt under militaristic principles will be a formidable match for the Soviets and Stalin might have to rethink his position. However Britain won't be able to do that much in manpower to the fight, and Germany will not have had the 'loot of west Russia' to fuel its war machine...something the Soviets do have.
Thus the Red Army will still be able to perform its miracle rebuilding during a time of war...and better than historically. This may lead to a very drawn out and protracted war against Communism.
Of course if Britain and Germany come to armistice, then Japan will have 'free hand in Asia' at Britians expense, on the other hand British, French and Dutch colonial holdings won't be at risk.
Japan makes an ideal ally for Britain and Germany in such a case to strike against the Soviets in the east and may get much British help there to realise the advance north scheme to invade Siberia and tap its mineral wealth...thus greatly changing the war in the east.
All said and done even with three of the Great Powers arrayed against the Red Army, long term the Soviets have the manpower and industry to bleed these other nations dry and the war could go on for much longer than in OTL.
In such a case the atomic bomb could be developed by an Anglo-German-Japanese alliance of expertise since all three powers were contemporary atomic research leaders.
America then gets a major wakeup call when in 1946 the stalemate in Poland-Ukraine is broken by the rising of a miniature sun on Earth. Heralding the near end of the war to end all wars (again).
With Europe mostly under autocratic leadership at this time, with only Britain, France and the Scandinavian nations not, you might have an early push for a 'European Union' since most of Europe has banded together to fight Socialism.
If the Europeans 'win', then Germany will gain Poland, Lithuania and Belorussia, while the Ukraine and Baltic states would be freed. Russia in Muscovy would still be permitted to exsit, but large tracts of Mongolia and East Siberia would be given to Japan.
If the Soviets 'win' then they might push for Poland, Iran and Turkey to join the Union as well as force the ending of autocratic regimes in Europe under threat of vaporising the historic and cultural significance of their major towns and cities to be replaced with cold, hard, scientific Communism.
All a bit of flight of fancy.