buzz,
i just saw that episode recently myself and i did notice that quentin called joan bennett "elizabeth".i haven't seen 1840 but i do know that bennett's character there is called "flora".
funny,i noticed something else during these episodes.i haven't heard "liz" in a long time.barnabas,julia and quentin call her "elizabeth".carolyn calls her "mother".david calls her "aunt elizabeth".maggie,mrs.johnson,hallie,sebastian and others call her "mrs.stoddard".
then it hit me that roger is not present during this storyline and he is primarily the one who called her "liz".it's sort of a brother/sister thing.most of the other characters speak to her more formally.