1. Literary elements and rhetorical devices are somewhat different because the purpose of an author's using them varies. For example, a fiction writer will often focus on figurative language, plot structure, and symbolism, while a rhetorician considers what his most effective appeals (logos, ethos, or pathos) might be or how he can best relay the truth.
2. Yes, many terms fit into both categories. Tropes are figurative language devices, and while readers/audiences most likely find many tropes (metaphor, imagery, puns, etc.) in fiction, rhetorical writers must also demonstrate adept use of the devices in speeches and essays). Schemes, which are patterns in sound, word, or phrasing usage, most often occur in rhetoric, but poets especially rely on the repetition of sounds and words (such as anaphora, alliteration, or epistrophe) to advance theme, meter, or rhythm).
I know that this can be confusing, especially if you are trying to study rhetoric for the first time, but keep in mind that if you are analyzing an author's use of rhetorical devices (such as the AP English Language exam requires), you can always incorporate an analysis of an author's tone, diction, or use of tropes, because effective prose authors/speakers employ the same literary elements that famous fiction author and poets use.