Have you read The Dead Hand? It offers an explanation for the fall of the Soviet Union better than any I've read; the old-schoolers up to Chernenko were all kleptocrats that understood perfectly that the sham that was Communism was supported wholly by the black markets, nepotism and the nomenclatura system while Gorbachev legitimately believed in Communism. So when Gorbachev instituted all these reforms so that Communism could work as intended, the whole artifice crumbled around his ears. It's a hell of a book. As the guy who turned me onto Richard Pipes, I'd be remiss in not recommending it.
My readings on the subject argue pretty convincingly that the reason Leninism had so many differences with Marxism is that Leninism allowed for an easier power grab and more opportunities for the nomenclatura to abuse the system. Further, "Leninism" was a moving target that changed according to the whims and needs of Lenin. It's not a far reach to say that the October revolution swept out the royal family and replaced them with a similarly aristocratic system of cronyism; the difference was the former had royal blood while the latter had street cred.
People also often forget that communism, as Marx envisioned it, was an evolution of capitalism, not a system that could be created out of nothing. Russia was not an ideal place for a communist experiment, because you can't redistribute wealth that doesn't exist. Lenin had his sights on Germany before WWI, but settled for his home country because, as you point out, he was an opportunist above all. This brand of opportunism was shared by Stalin and his team. His foreign minister, Molotov, once remarked, "our ideology stands for offensive operations when possible, and if not, we wait." I would guess Putin is familiar with this line of thinking.
I was told that Lenin was also trying to "kick start" communism. He didn't want to wait for Marx's evolution of capitalism to happen. Marx, as it was explained to me, basically said things had to get as bad as capitalism could get before the revolution occurred and communism as he saw it came into existence. Lenin wanted to avoid that, mostly because he wanted to see it in his time.
Marx had correspondence with the early partisans in Russia. He warned them that they were doomed to failure, and that he wouldn't support their cause. Apparently, he eventually gave them approval, but in a kind of reserved way. Lenin would have been keenly aware of this, but I suppose he decided that either he was smart enough to figure it out, or his ego was such that he really only desired the personality cult. Probably both. Whatever the case, the failure of Russia in WWI was the perfect opportunity to seize control, and it was actually Germany who was responsible for transporting Lenin from Switzerland, where he lived in exile, back to Russia. To them, it was a type of alternative warfare to bring down the czar from within. It's a great lesson in figuring out if the sure is worse than the disease before implementing it.