¥eah, i agree that imaginative is pretty hard, and pretty risky too because your mark will partly be based on if your teacher likes or dislikes your style of imaginative writing.
I think that you should decide between persuasive or expository depending on the prompt you are given. If the argument is pretty one sided, then it will be easier for you to do persuasive. If you can think of many points argue for and against, it might be a good idea to do expository.

Ohh, and make sure you have a good understanding of how Catcher in the Rye relates to the topic Identity and Belonging. Although you don't know your prompt yet, you can pretty much write up bits and pieces that you *might* be able to include into your essay when doing the sac, but just make sure you are still addressing the prompt. I lost many marks on my sac because i didn't address part of the prompt properly

You can pre-prepare by writing up some arguments for or against certain points with supporting evidence from the text. And make sure you know some basic stuff about Identity and Belonging, maybe some quotes and stuff, because your entire essay doesn't necessarily have to be based on the text. You can use other relevant info too if it relates to the prompt . And even if you cannot use them in your actual sac, it will still be good practice.