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.