First question: The backstory gives the audience information about the events that occurred before the play began.
Second question: The main purpose of a backstory is to explain what has happened before the story.
The sentence that has the correct subject- verb agreement is the following one: Neither the Western Hemisphere nor the Eastern Hemisphere are immune to flooding.
The verb "to be" can't be singular because the writer mentions the two hemispheres (the Western Hemisphere and the Eastern one). That is why the first option is the correct one.
