Sorry, I was writing from memory. Having checked, these half shares were agreed, then revised (as you say) on the proviso that the USSR would attack Japan. However, I still maintain that without these double agents, there could have been a better situation. The US forces in Southern Germany and Austria could have entered Czechoslovakia, possibly even beating the Red Army to Prague. The reason they didn't was due to politics and diplomacy. Maybe the negotiations would see an 80/20 (to the west) share in Czechoslovakia being negotiated, with 50/50 being the pragmatic result due to the disposition of land forces. IIRC, western troops were also slowed/halted at the Elbe, again due to diplomacy.
It is true that the widespread infiltration of Soviet spies and Communist sympathizer traitors in the ranks of the British government and the Roosevelt administrations helped create the unfavourable political-diplomatic ground you describe, however indeed as you acknowledge even without changing anything else in German military performance, the Western Allies could have achieved military demarcation lines rather farther in the East with a bit more daring, and from such better ground, more favourable results from diplomatic bargaining would have ensured. It is wholly feasible for the Anglo-Americans to conquer Berlin, Vienna, and Prague first, and place the demarcation line on the Oder-Neisse or the Oder, the Czech-Slovak border, and the German-Hungarian border.
Sparing more of Central Europe than that from Soviet control is very difficult without a 1942-44 PoD, but the occupation zones, territorial changes, and spheres of influence were not really defined till Potsdam. Such a military border would in all likelihood result in Czechia, Slovenia, and Croatia staying in the Western bloc (Stalin never tried to challenge Western control over areas that the Western Allies occupied until the Cold War was in full rage) and the Soviet occupation zone in Germany and future East Germany being restricted to Pomerania and Silesia. East Prussia might be part of East Germany as well or rather more likely be given to Poland as sole compensation for loss of Eastern territories to Russia.
It is also quite possible that Stalin, having made more limited gains in Germany and Czechoslovakia, decides to recoup by annexing Poland (with East Prussia) to the USSR, and by breaking the armistice with Finland to conquer it and make it a Communist satellite or quite possibly annex it to the USSR, too. In such a case, Polish SSR would likely keep Lwow and Finnish SSR all of Karelia, and keep those territories when the USSR collapses. Given the basic demographic realities, it is wholly unlikely that Stalin or his successors would be able to achieve large-scale Russification, there would be some Russian immigration but not going to more than 5-10% of the population. It would be a tragic crucible for Finland, not really that different for Poland, both countires would recover theri independence like the Baltics and Ukraine when the USSR collapses. Communist or SSR Finland surely means Sweden in NATO and Eu from the start.
It is questionable whether a Yugoslavia deprived of Slovenia and Croatia, effectively a Communist Greater Serbia, would be able to affirm its independence and neutrality from the Soviet bloc, or Stalin would be able to crush Tito. If the Soviets manage to keep "Yugoslavia" within their bloc, it is also quite likely that Macedonia would receive a different settlement, perhaps an attempt to create the Balkans federation around it with Yugo-Serbia, Bulgaria, and Romania, maybe followed by partition between Albania, Serbia, and Bulgaria, and/or a Bosnia-like ethnic conflict when the Soviet bloc collapses. If Yugo-Serbia stays true to Soviet allegiance, the Greek Civil War would last longer but eventually end in Western victory all the same.
The Eastern border would be slightly more favourable to Italy if the Anglo-Americans occupy Slovenia and Croatia, Italy would keep all the OTL Free Territory of Trieste from the start. The Soviets would likely try to make half-hearted attempts to lure Czechs and West Germans to national unity with Slovaks and East germans in neutral states, however, links between Czechs and Slovaks not being that much overwhelming, and West Germany being the vast majority of the nation, such attempts would most likely fail. With the Western occupation zones encompassing Saxony, Brandenburg, and Austria, too, it is quite likely that a separate Austrian consciousness never develops. The unification of the various Western occupation zones in Greater Germany is pissibly delayed a few years owning to Western concerns about recreating a stronger united West Germany. However eventually overriding Cold War concerns about using Germany as anti_Soviet bulwark, evidence that West Germany is firmly kept into rein by the Western political-security NATO/EU system and the presence of Allied troops, overcome such fears. When the Soviet bloc falls, Greater Germany reunifies, only East Prussia being lost for good. Czechia and Slovakia stay independent, so do Slovenia and Croatia, which are spared the post-Yugoslav wars, which sweep Bosnia, Kosovo, and Macedonia.
The Western bloc ITTL is definitely richer and more powerful, with a much larger and stronger West Germany with OTL East Germany and Austria, Czechia, Slovenia, and Croatia, not to mention Sweden in NATO and EU from the start, and the Soviet bloc definitely weaker, with an East Germany only made up of Pomerania and Silesia, and no Czechia, Slovenia, and Croatia, even if it may recoup losses to a degree with loyal Yugo-Serbia and direct control of Poland and Finland. Likely the fall of the Soviet bloc is accelerated by 10-15 years. All in all, a net gain for Europe and the Western bloc, even if Poland and Finland may easily pay the bill for Soviet frustration.