I've been in a similar position before with teaching texts that are clearly far more complex than they overtly appear to be and then having to find a grade appropriate essay question. A great way to do this is to give a question that is as open as possible and gives the students the chance to interpret the text based on that quotation at their level. One of my favourite quotes from this story, that you could easily use to form the basis of an essay question, is the following:

What's the use of stories that aren't even true?

Clearly, this excellent novel is about the value of stories and their purpose in life. Using this quote as the basis of an essay question will allow students to discuss what the novel presents as the answer to this question and then also their own response to this statement. This will allow them to show their knowledge of theme and its importance whilst also having to argue their own point. I hope this helps!