The inverse triangle method is just starting with a general statement and making each sentence more specific for example:
I would rename the method "The Increasing Specificity Method"
But im not sure if this essay is supposed to have a theme in it...
All essays need a theme. The essay is something you are supposed to have written because of sheer inspiration -- even though you are actually writing it because of an assignment. Write it as though you have one big idea about the symbols.
Frezard is right, this is SO helpful: sparknotes.com/lit/huckfinn/themes.html
And as for your paragraph writing method, it sounds like you have become too mechanical in your writing due to the way you were taught composition. Free your mind. You are allowed to make your own original observation about the symbolism. You can even look at the symbols from a unique perspective; for example, look at the symbols by considering their implications for a 21st century adolescent.
I was learning about several methods of paragraph development recently. Contrast, Example, Cause-Effect, Definition.
Google around about "paragraph development"