Answering questions

1) If Hitler hadn't broken the Nazi-Soviet Pact and hadn't launched Operation Barbarossa, the UUSR would've probably fought against the Allies, right?
Not really. If the pact hadn`t been broken the USSR & Germany would obviously not have fought one another. However, Nazism & Communism were as ideologically incompatible and the same way as capitalism and communism were. Stalin did not want to fight at all. In fact, he distrusted UK & France because he thought they expected USSR & Germany to tear each other to pieces in the East. This is sth he did not want.

2) Cold War: how could the Policy of Containment be carried out? We understand that the US wanted to prevent the spread of Communism, so the US adopted two approaches, right? One approach was military, the other was economic. Was the Marshall Plan the economic side of this Policy of Containment?
If by military you mean the US actually fought, with the exception of Korea & Vietnam the US did not actually fight. Never the USSR openly. If by military you mean providing military aid, personnel and advice to other countries the answer is yes.
Aid to Turkey and Greece, as promised in the Truman doctrine is economic, so is ERP (Marshall Aid).
3) WWII: Where did the Germans take the money from to rearm if they were bankrupt? Did the US provide loans to Germany? Germany was forgiven the payment of reparations. In 1923 the Dawes Plan eased conditions for Germany. So did the Young Plan 1929-30.

4) Which was the difference between Lenin's and Stalin's aims? Did Lenin want to spread Communism all over the world, and did he want the proletarian revolution that Marx & Engels had written about in, for example, the Communist Manifesto? It’s not a question of what Lenin or Stalin wanted, it’s a question of communist ideology which is by definition revolutionary and international in character.
Was Stalin a nationalist? No he was not. Communism and nationalism are ideologically incompatible. Nationalism is a bourgeois ideology from the French Revolution. Communism is internationalist. In order to consider the initial world nature of the Bolshevik Revolution you should find out what Trotsky’s aims were.
Wasn’t Stalin interested in spreading Communism to other parts of the globe? It’s not a question of his being interested or not. It’s a question of what the USSR was able to do. Reread Hobsbawm when he explains how the Bolshevik revolution took place in a country that was not manifestly ready for it according to Marxist theory. Stalin proposed the slogan “Socialism in one country” to indicate the fact that, for the time being, the USSR would not seek to spread revolution across the world. Hence his split with Trotsky

5) How did the Russian Revolution save the USSR from the Great Depression? Was it because of the Five Year Plan implemented by Stalin? Wait a second, the Bolshevik Revolution took place in 1917 and the Great Depression started in 1930. When the Slump started the Revolution was over. What “saved” the USSR was the fact that it did not have a capitalist economy. The Five-Year Plans were merely the expression of a communist economy. Think in terms of theories and ideologies.