... I doubt that he would even be a Marxist ...I think you're absolutely correct. I think Marx had enough integrity as a philosopher and as a revolutionary to review and even revise his findings in light of new facts. He did it all the time throughout his career.
America's Left is no different in aim or outcome. War is coming...
"The Party is all-embracing. It rules our lives in all their breadth and depth. We must therefore develop branches of the Party in which the whole of individual life will be reflected. Each activity and each need of the individual will thereby be regulated by the Party as the representative of the general good. There will be no license, no free space, in which the individual belongs to himself. This is Socialism - not such trifles as the private means of production. Of what importance is that if I range men firmly within a discipline they cannot escape? Let them own land or factories as much as they please. The decisive factor is that the State, through the Party, is supreme over them, regardless whether they are owners or workers. All that, you see, is unessential. Our Socialism goes far deeper..."
"The people about us are unaware of what is really happening to them. The gaze fascinated at one or two superficialities, such as possessions and income or rank and other outworn conceptions. As long as these are kept intact, they are quite satisfied. But in the meantime, they have entered a new relation; a powerful social force has caught them up. They themselves are changed. What are ownership and income to that? Why need we trouble to socialize banks and factories? We socialize human beings."
Adolph Hitler - Rauschning, The Voice of Destruction pp 191-193
True, he did keep his theory a "moving target." But that was not to accommodate emprirical test or data, but to avoid them.
As to his integrity, read "Marx: The Red Prussian" based in large part on his correspondence with F. Engels.
He apparently was neither a nice nor an ethical man.
In fact, according to the author, Leopold Schwartzschild (sp?), his nickname in his graduate student days was "the calf biter"!!!!
My reading of "On the Jewish Question" also sees it as antisemitic (self-hating?) and positing an identity between "Judaism" and "Capitalism."
Much as Weber later linked "Protestantism" (The Protestant Ethic)with (the Spirit of)"Capitalism."