How do the elements in "A Rose for Emily" forward the plot and establish the atmosphere?
In "A Rose for Emily," one element that is central to the plot and atmosphere is the collective narrator, who represents the townspeople.
The townspeople, serving as the narrator, reveal information in a haphazard (seemingly) manner, looking at Emily and the situation from the outside, in. This is central to the story.
This enables plot to be revealed in a seemingly random order, and in bits and pieces, as if an outsider is looking in and doesn't know the whole story. Thus, the reader does not make the connection between Homer's disappearance and his murder by Emily.
Obviously, the surprise ending could never be accomplished if Emily, her servant, her father, or Homer were the narrator. Anyone on the "inside" could not ethically conceal Emily's murder of Homer. Or, if we're looking at all possibilities, if Emily were the narrator and kept the murder hidden, the story would be drastically changed into a psychological study. It would not be the same story.
The plot and the atmosphere depend on an observing narrator who observes from outside the house. That is central to the plot and the atmosphere. The outsider narrator observes the real outsider, Emily. For the eerie, confused, unnatural atmosphere of the story to exist, and for that atmosphere to culminate in the hideous final scene and surprise the reader, the narrator must be outside the home.