Trotsky was a true believer until the end. But much like Marx and Engels, while he had a solid grasp of the economic underpinnings of communism, none of them actually had even a decent grasp of the political underpinnings. Nor did any of them have a decent grasp of the sociological foundations necessary to determine if communism would work - but then no one else did either since sociology and group psychology did not come into its own until the same era, and has never settled on any grand scheme similar to the mainstream schools of economics and political science.
Lenin believed in communism, but that the vanguard had to lead the proletariat until they were ready for self-governance - yet the only one who actually encouraged any steps toward that was probably Trotsky. Lenin’s primary concerns where industrialization and building the party into a coherent, ideological cadre. Yet he was soon co-opted by Stalin, who was only interested in totalitarian dictatorship, for which communism was just a rationale, never a purpose.
I think Khrushchev believed in Leninism, and that communism would be a viable alternative to capitalism. Brezhnev and the other hardliners were more nationalistic and wanted a strong military-industrial state to counter generic Western imperialism, and had no problem with an authoritarian government to accomplish those means.
As for your extra credit question, I would say no one of influence still believes in Marxist-Leninist communism. As research progresses in sociology, political science and economics, I cannot think of any serious professor or politician that still believes Marxism is a realistic system. Mainstream socialism has moved beyond Marxism and is more concerned with reforming the market-based capitalist system through democratic means, then its revolutionary overthrow, and few believe that socialism is part of a continuum that leads to communism.
A few professors are still* Marxian*, i.e. believe that Marx’s dialectic materialism and economic theories are valid forms of analysis. They are found more in sociology and history departments than poli-sci departments and very few left in any econ department.