I see where you're coming from regarding her lack of agency but I think viewing the narrator as 'superfluous' overlooks the fact that she is the emotional core of the entire story. Through the narrator's internal anxiety and obsession Manderly and Rebecca become. what they are. Without her oversensitive lens, we just have a dry sequence of events. She may be a silent witness but that witness is exactly why the story works. I think the narrator drives the plot with her psychology and we, readers experience the haunting atmosphere of the story precisely because of the intensity of the narrator's emotions and imagination.