just a thought but perhaps the collins family and the other "normal" characters were supposed to represent "us" the viewing audience.
barnabas and julia knew all but the rest of the characters,like us,gained bits of information on an episode-by-episode basis.because the situations were so preposterous they just didn't see the "big picture" and couldn't put the pieces of the puzzle together themselves.then,as i said earlier,once that particular crisis was over with they acted like it had never happened or were somehow influenced so that they "forgot" it.
my feelings on professor stokes was that he was certainly aware of the fact that something highly unusual was always going on at collinwood but i never got the impression that the whole 'vampire' thing had fully fleshed itself out to him.especially since during much of their interaction barnabas was not actually in the vampire state.