Seems like a stretch but I'm familiar with the points you make and I appreciate the connections. I find it hard to believe that Russell was familiar with this particular bit of trivia, that Marx sent Darwin a copy of Das Kapital (which Darwin never read) and to do so would somewhat rehabilitate Malthus who Russell seems to want to associate with Marx -- transferring Malthus's disproven theories onto Marx: one of the leading scholars responsible for thoroughly disproving them.

I still feel that within the first few chapters of Capital and definitely in his other works, Marx pretty clearly diverges from his influences of Malthus, Ricardo, and Smith. Stating that they all were touching on the truth, but missed it in some way because they didn't understand the primary contradictions of capitalism. Which he is understandable about as many of them were writing during the reign of kings and hadn't yet seen the monster that was capitalist imperialism.