>1. No nation-level Communist regime has ever been instituted organically, that is, without vanguardism.
On the same token, no nation-level Communist regime has ever been instituted with vanguardism. They never claimed to bring about Communism, only that they were "socialist states". Nevertheless, it is important to distinguish Leninism as incorporating ideas from Lenin from Marxism-Leninism, an ideology created around Stalin's systematizing as a state ideology.
The truth is that I don't know what it would take, especially in today's world. I'm pessimistic about the possibility of workers as a whole uprising, and in the case that they did, succeeding. As David Schweickart wrote on his essay on Sartre's Marxism:
>Moreover, the democratic process, our best hope for relatively peaceful social change, is, increasingly dysfunctional, due in no small part, to the fact that, since the mid-70s, there has been a coordinated, beneath-the-radar, well-financed, largely-successful, ongoing effort, not only to keep “Democracy in Chains,” but to tighten those chains more and more so as to shield the wealthy from democratic discontent. Not good news for Marxists hoping to “win the battle of democracy.” At the same time the prospects for a violent revolution by progressive forces (as opposed to a right-wing military coup) are essentially zero. (A successful storming of the White House? A guerilla army swooping down from the Appalachian Mountains?)
But the point is this: by going through an intellectually lazy route to say that Marxism died with the collapse of 20th c. Communism, we are already shutting down discussion of what Communism could be. The hypothesis, from my point of view, has not been refuted. It is less a matter of teasing out Marx's vision of a future society (which is lacking, because of his philosophical commitments) but rather trying to see what a future society would look like based on the critique. Unlike the diagnosis from a doctor, the features of a socialist society flow from the critique of capitalism, indeed, I would say they are already within it.
I am no dogmatist. If we find that Marx's critique must be reformulated or even abandoned, then so be it. But I think this is less and less the case when Marx's framework is being fruitfully used today for apparently modern phenomena, from McDonaldsisation to the neoliberal state.
So to answer your question: For Marx to be wrong on the solution to capitalism would, for me, necessarily entail that his critique is lacking or wrong. Since we are more able to access the latter than the former, we can sort things out from there. This is actually a weak claim - because I admit that Marx's vision of socialism depends on his critique. A stronger, dogmatic, claim would be that even if Marx was wrong about capitalism then his socialism would still be the answer. I don't think that's defensible.