Thanks all for the hints on the middle square. I assume the connection is art here? I considered this before but rejected it because I missed the Caroline connection.
I'm still not sure how we're supposed to fill the fourth blank square. I'm guessing a punctuation mark and the dodgy equivalence sign.
I thought the overall idea for the puzzle was excellent, and items 1, 2 and 5 work nicely. The other two are, as others have said, somewhat GWITtish (in my view).