Your statements about Stalin are conveniently ignoring that Stalin was not the leader of the revolution, only its butcher. You should not for one moment believe that Stalin was not committed to socialist revolution. If he were only interested in power, he would not have needed to finish off NEP and plunge the country into turmoil. It would have been safer to continue the policies put into place when the true revolutionaries saw that things weren't working like they had hoped.

Instead, Stalin consigned millions of peasants to a torturous death so that he could industrialize the nation in keeping with Marxist dogma about the development of communism on the base of a highly developed industrial nation. Urban workers were time and again given preference over the "counterrevolutionary" peasants. They were trusted (given internal passports that let them move around the country) and it was from this industrial worker base that the Party tried to draw most of its recruits.

Stalin was a socialist. Being a socialist and at the same time being paranoid to the point of bloodthirsty murder are not only not mutually exclusive, but actually quite common.

Mao was not a traditional Marxist. There is a reason that "Maoism" is considered a separate doctrine from Marxism-Leninism, even though Mao claimed to follow the latter. It is because Mao tried the orthodox Marxist path to power and was quickly crushed by Chiang Kai-Shek because there was no industrial base in China. He switched to a model that saw the peasants as the force for revolution and was successful. However, despite some terrible campaigns designed to bring industry to the peasantry, like the misnamed Great Leap Forward, Mao remained a peasant-oriented dictator and continued along that path.

But that's not where you stop being wrong. The point is that both Lenin (and, after him, Stalin) and Mao realized that the conditions which Marx set out for communist revolution did not exist in their respective countries. Lenin tried to argue that Russia was "close enough" by writing an entire thick book (wholly unreadable) titled The Development of Capitalism in Russia (Развитие Капитализма в России). Stalin followed this path and developed (by stealing the Bukharin platform) "communism in an individual state" without formally renouncing world revolution. Mao essentially argued that China was a special case because the peasantry had been so oppressed by their landlords that they were China's equivalent of a proletariat. All of these leaders recognized that criticism could be made of their doctrines and all responded in an informed way.

Not only that, but history's verdict DOES invalidate communism. Each experiment in socialism that approached communism killed more and more people the closer it got. Pol Pot probably came closer than anyone else to implementing full communism in a nation-state, and ended up killing 25% of the population to do so. The scale of murder undertaken was so colossal that many Cambodians of younger generations can't even believe it. We don't need to see an actual full communist state attempted to know that it won't work, and history helps us to prove this. Each time a socialist country has tried to approach communism in one way or another, the result is mass murder, famine and turmoil. So yes, history DOES pronounce a verdict on communism.

We might also have a discussion about whether communism fails because of "human weaknesses" or because of "human strengths", but that is a relative argument and I wanted to stick to points which you made that can be rebutted factually.