> Stoker could, for example, have sent Valdimar an older version of his story

Back in the old days, I could see happening as purely a mistake and considering how global communication worked then I can certainly see this not noticed for quite some time. Stoker's publisher could have grabbed the wrong manuscript and sent it out. Considering the Amazon reviews claim that the second half of the book is, at best, a rough outline, then I'm guessing this is probable. It sounds like an unfinished and abandoned work that somehow got into production.