My largest problem with the Dark Lord == Death theory is that it doesn't really square with Quirrelmort being another super-rationalist and Eliezer's First Law of Fanfiction (You can't make Frodo a Jedi unless you give Sauron the Death Star). Either Quirrelmort is a henchman or personification of Death, which is unlikely considering he is afraid of dying and the dementor try to frighten him in the Humanism arch. Or Quirrelmort is not the Sauron of this story but will help Harry to defeat the main bad guy Death. This could be a really cool ending, but I doubt that it would fit in the remaining arch.