I always got the impression that being Jewish was a rather incidental thing for Marx, and that he had a problem with the Jewish population identifying themselves ethno-religiously in that it was a barrier to the development to developing class consciousness among the working class.
If you actually read the Communist Manifesto (as you claim you have), you would recall the bit about how the working class has no country or nation.
I have no idea where you're getting these ideas that he was a Jewish nationalist.