How does Faulkner use the physical descriptions of Miss Emily, her conflict with the townspeople, and the revelation of the story's final paragraph...

These type of descriptions that you are asking about fall under two categories:

Direct characterization and Indirect characterization.

Faulkner uses both cleverly by juxtaposing the direct description of Emily, in contrast with her actions. He does this primarily to deflect the attention of the reader and, ultimately, cause the big shock at the end.

Direct characterization happens when Faulkner gives us a step by step account of what Emily really looks like, for example: 

[Emily...

These type of descriptions that you are asking about fall under two categories:

Direct characterization and Indirect characterization.

Faulkner uses both cleverly by juxtaposing the direct description of Emily, in contrast with her actions. He does this primarily to deflect the attention of the reader and, ultimately, cause the big shock at the end.

Direct characterization happens when Faulkner gives us a step by step account of what Emily really looks like, for example: 

[Emily is a] small, fat woman in black[...] Her skeleton was small and spare; [...]She looked bloated, like a body long submerged in motionless water, and of that pallid hue. Her eyes, lost in the fatty ridges of her face, looked like two small pieces of coal ...

This direct characterization of Emily is meant to show that the woman whom so many in Jefferson County feared, and thought to be so enigmatic, is merely a short, plump lady of old age whose demeanor may seem unhappy, but is otherwise harmless. 

It is almost as if Faulkner is asking the townsfolk "What is wrong with all of you?" "Can't you see that this little lady is incapable of causing harm?" We know that he does this on purpose, as Emily's actions will proof to be anything but harmless. 

Faulkner therefore uses indirect characterization to contrast the image of this plump, old lady against the absolutely strange things that she does. This builds up for the ending to come ore dramatically.

An example is the time when Emily refuses to give up the body of her dead father for burial because she was in denial that he was dead. Another example is when Emily suddenly starts dating a drifter from the North, Homer Barron, who is the last individual anyone would have ever thought that Emily would be interested in dating. A third example is when Emily looks out the window as the people from town throw limestone at the surroundings of her house, given that a stench that came from the house was so bad that the residents were bothered by it.

All of these are the actions of someone who is quite strange; someone who does not seem to be congruent with one action and the next. This, when contrasted to the otherwise harmless look of Emily, gives the reader a sense that, with this lady, anything could happen. 

Finally, when the reveal that takes place in the end verifies the previous assumption. The reader realizes that, indeed, anything can happen with Emily. The revelation that Emily had lived with Homer's dead body, and that she is likely who may have killed him, makes the reader revert back to all the instances of direct and indirect characterization, in order to connect the dots and put all the details together. 

Therefore, Faulkner juxtaposes indirect and direct characterization as a way to show the contrast between what Emily looks like and how she behaves. This helps to foreshadow the big surprise that will come at the end of the story.