What are some examples of metaphor, understatement and hyperbole in To Kill a Mockingbird chapters 12-31?
In Chapter 13, Scout discusses how old Maycomb is, calling it "ancient." She also says that the first tavern in the area was founded "in the dawn of history." Scout is clearly exaggerating (hyperbole); although Maycomb is old, it isn't ancient (this would be something like 100 B. C.) and the tavern began much more recently than the dawn of history.
But Maycomb would have been closer to the river had it not been for the nimble-wittedness of one Sinkfield, who in the dawn of history operated an inn where two pig-trails met, the only tavern in the territory.
In Chapter 16, Atticus talks about the mob that confronted him at the jail the night before. When Scout asks about Walter Cunningham Sr., Atticus says he is basically a good man but has his "blind spots." He doesn't mean this literally. He means that Walter has some flaws. This flaw happens to be the inability or unwillingness to rise above racist and/or simplistic thinking about Tom Robinson. Blind spots - a metaphor describing mental faults, not actual sight problems.
At the beginning of Chapter 23, Miss Stephanie relates the story of how Bob Ewell spit on Atticus and challenged him to fight. Atticus replies that he is too old and calmly walks away. Miss Stephanie says he could be "right dry" sometimes. This is a bit of an understatement. He is not just replying in dry conversation; he has a zen-like calm, focus, and understanding of the situation.