In Imperialism, Lenin is specifically trying to outline the primary contradiction in the world as he saw it as it evolved from Marx's era, as an explanation for why Revolution hadn't yet occured in developed Capitalist countries as Marx had initially thought. Rather than the most developed countries, Imperialism causes revolution to happen in the Global South. This came with a whole host of new questions, but those aren't going to be answered in Imperialism. Instead, you'll find discussion of revolutionary strategy in The State and Revolution, as well as What is to be Done?

As for "authoritarianism," the standard Marxist stance can largely be found in Engels' On Authority, which is a very quick read. I would also recommend Blackshirts and Reds by Michael Parenti if you want to really understand Marxist movements and why the vast majority of Marxists support them, even if they aren't utopian wonderlands. You don't have to agree, but you'll have a much better understanding of your Marxist comrades that way.