As detailed very well by Tooze in “The Wages Of Destruction”, there was a simple reason for the USSR to support the MR pact. It had the potential to upend the balance of power in Europe. Doing the opposite and siding with the western powers would only reinforce a status quo that Moscow already found stifling and dissatisfying. From a purely material point of view, Germany would always have more to offer. This was explained in cold logic to British ambassador Cripps by his Soviet colleague who said that Britain would count air losses in the Battle Of Britain as two opposing columns of entries, whereas the Soviet Union would just add them up.
Also at the same time, supporting the weaker party in the war - Germany - fostered inter capitalist war. This made it less likely that Stalin’s nightmare scenario of an anti communist alliance between the western powers and Nazi Germany would ever materialise.
It’s also important to note that the MR Pact always operated on the spirit of bilateral imperialism, with spheres of influence clearly defined between the two powers. In that sense, yes, Finland and the Baltics were assigned to the Soviets by the pact. Initially Lithuania was supposed to fall in the German sphere, but this was later exchanged to the Soviets in return for a larger share of central Poland to the German “general government” administration in Krakow.
Both sides had very good reason to pen the deal when they did, but it’s important to keep in mind that over time, the economic aspect of MR was making the Soviet position relative to Germany stronger and stronger. Hitler always intended to break the pact, but one of the reasons it happened with the timing of OTL is because Hitler understood that the pact would soon start benefiting the Soviets more than the Germans, if it wasn’t already.
Hitler never cared about Danzig specifically. At multiple times during his tenure in office he expressed his disdain for the way German nationalists would fixate on, for example, the borders of 1914. Symbolic lines on a map were not what Hitler was after.
Ever since the very first draft of Mein Kampf, Hitler’s interpretation of the balance of power was this: the European powers were being eclipsed by the “flanking powers” (USA and USSR) because the latter had been able to create territorially contiguous colonial empires, which were immune to blockades, economically self sufficient, and with an internal market large enough to hit critical mass and benefit from mass production in full.
Hitler believed that the only recipe for Germany to compete was to carve its own contiguous colonial empire in Europe, and that obviously this would have to be in Eastern Europe, which naturally implied genocidal policies on a vast scale towards the local population. When Hitler made his famous quote about “who remembers the Armenians today”, he wasn’t speaking about the Jews, but about the Poles.
Maybe even more importantly to the Danzig argument, the logic of rearmament put massive pressure on Hitler. He was perfectly aware that Germany did not have the industrial resources to compete on an even footing with all of its enemies. It had an initial advantage because of early recovery from the Great Depression and early rearmament, but Allied rearmament after the fall of Prague in March 1939 was going to erode that advantage, so it becomes a matter of “use it or lose it”. Which is why Hitler greatly accelerated his plans, and over the summer of 1939, would tell anyone who would listen that Germany had nothing to gain by waiting.
When I was still living in Germany, I had the privilege to directly research the development of the Nazi economy through both primary and secondary sources, so there’s a lot that I could recommend in terms of reading material, but I think Wages is a really good place to start. Imho still the best book on The Thord Reich I’ve read by any historian.
As for War Thunder, the only reason Sweden is in the Allied camp is to manage queue times for simulator. It’s sad, but true, and unless someone has a serious solution to address that player imbalance issue, that’s unlikely to change any time soon.