So it's almost like Archie psyched himself into thinking of Chapin as this serial killer/vampire creature to the point where, when he met the guy, he even *looked* eerie and otherworldly (is "Creepy Albino" on tvtropes? I suspect it is) but then once he knows the whole thing was a hoax, all of a sudden Chapin even *looks* different. So I really wonder how much of what comes earlier is trustworthy and how much is colored by Archie's perception of the Chapins as creepy serial killer couple of the year.

Well spotted!

Archie admits at one point earlier in the book that Paul Chapin has him completely spooked, and he even goes tongue-tied and tense when he comes back to the office with the typewriter and finds Chapin sitting there which, come to think of it, never really happens in any of the other novels. People will frustrate and annoy Archie, but I don't thing there is any other character who manages to get under his skin and creep him out like that.