How can I make an original argument about the use of speculative elements in either Josiah Henson's narrative or Flight to Canada?

Your first strategy in completing this type of assignment is to think about constructing a theoretical framework. One possible approach would be to start with the theory of the "uncanny" as originated by Freud and developed by Kristeva as a theory of "abjection."
The two scholarly resources you could use for this are:
Freud, Sigmund. "The Uncanny." Trans. Alix Strachey. The Norton Anthology of Theory and Criticism. Ed. Vincent B. Leitch. et al. 1st ed. New York: W. W. Norton &, 2001. 929-52.
Kristeva, Julia. Powers of Horror: An Essay on Abjection. Trans. Leon S. Roudiez. New York: Columbia UP, 1982. Print.
You could focus on the episode in which Mitchell Swille's ghost and then Swille's sister appear. Both of the characters function to undermine Swille. Thus the uncanny serves as a destabilizing element in the story and a site of resistance to Swille. Kristeva's notion of abjection, and particularly the uncanny return of the past, is especially relevant to these ghosts from the past that serve to destabilize the patriarchal repression of Swille's wife by Swille. Thus in a sense you could argue that Reed is embodying Kristeva's notion that horror and the uncanny are the "others" cast off by patriarchal and racist society but always remaining to subvert it.