It was left out in the movie, but in the novel, Plato’s The Republic is explicitly mentioned, it is read by Sonmi 451 in the process of her “ascension”. The society she lives in is also based on “castes” (similar as in the other Cloud Atlas stories/episodes), purebloods and fabricants (clones), and founded on a lie (the promised retirement for fabricants – instead death awaits them), like a warped version of the Noble Lie.
Then you find a civilization’s rise and fall as in Plato’s myth of Atlantis (in Timaeus) and reincarnation of souls (that our choices and the character we develop will have consequences after death) as in Plato’s myth of Er.
Maybe those are more promising starting points to investigate Plato’s themes in Cloud Atlas.
Now, regarding the allegory of the cave: its point would be seriously lost, in my view, if you reduce it to “reality turns out to be dramatically different than you assumed it to be” – it’s first about Plato’s theory of Forms.
But if you have to make a connection, one could compare Sonmi’s life in the restaurant to one of the cave dwellers. She has a totally false view of reality, created by a corrupt authority.
Similarly, the Valleymen’s understanding of reality is extremely limited compared to the Prescients. Zachary’s views on Sonmi are completely false. He regards her (in fact a historical figure, leader of a revolution) as a Goddess. Even in the observatory, when he sees the Sonmi statue, he just asks: “The Old Uns pray to Sonmi same as Valleysmen?”
I’m simply at loss what “Aristotle’s monism” should mean in this context and I can’t recognize any major Aristotelian themes in Cloud Atlas – sorry.