Agree with what you write on historical fiction, though I'd defend psychology: history is not made by figures, but living people. Though definitely not everything is a romance. As for "Marxism in the eighteenth century" problem, I think it can defend itself to some point if the author refers to pre-Marxists tendencies 18th century philosophy, as nothing comes out of nowhere (I'm not a specialist in the period, but theoretically it can be done; my master's degree thesis was basically on pre-Freudism in 19th century Polish fiction), but it's another matter and not really relevant here. But I generally agree, if the literary events take place in particular historical period and/or particular place, it should be made for a reason and with careful modifications for probability, as well as characters' psychology.