1) I love both alternate and RL-historical fiction, in that order, and I'd have to say one of the prime rules of writing both is "know the rules, so you can figure out how best to break them, if needs be".
For best effect, novels should remain true not only to the known historical facts or tweak them in an understandable direction (or complete throw them out the window and do a merry dance in the room left behind), but must also have the flavour of the "look and feel" of the times. You can break one or the other, but not really both at once, even in alternate history, unless you're very, very good and manage to convince the reader that you had good reason for doing that.
Incidentally, I think the romance term for those books where the Regency/whatever setting is just a blue-screened backdrop sloppily pasted in behind the story is "wallpaper historical", a slightly derogatory term.
2) Maybe I've just watched the TV show too much, but "Dexter" as a protagonist's name kind of implies that said "atheist intrigued by the new sciences of the age" is perhaps getting his practice in them by vivisecting, discreetly and conscientously disposing of those indulging in less-than-gentlemanly vices in a way that would cause Society to gossip, while pursuing his soon-to-be-widowed lady-love, Marguerite, mother of daughter Aster and son-whose-name-doesn't-translate-well-to-Georgian-times.
That would be a cool mash-up, though.