Why does Emily Brontë choose to tell Wuthering Heights the way she does, and how does this possibly affect the reliability of the narrators?

There are three narrative levels in "Wuthering Heights"

1. Primary: The dates 1801 and 1802 in Chs 1 and 32 clearly indicate that the entire novel is a written record of all the incidents narrated to Lockwood by Nelly Dean. He is thus both the primary narrator and the primary narattee. The method of narration is the first person past written method.

2. Secondary: Nelly Dean is the secondary narrator who narrates all the incidents to Lockwood. The method of narration is the first person past/present spoken method. Most of the incidents she narrates  have already taken place, but when she reports the exact words of a character especially during an intensely emotional scene (Ch11) Emily Bronte creates the illusion that the incident is happening just then.

3. Tertiary:  Some of the incidents are first narratred  by the different characters first to Nelly the secondary narrator who in turn narrates them to Lockwood the primary narrator: Heathcliff's oral accounts in Chs6 and 33; Isabella's letter in Ch13 which is read out aloud to Lockwood thus, combining the written and the oral method; Isabella's oral account in Ch17 ; younger Cathy Linton's oral account in Ch24; and Zillah's oral account in Ch30.

A first person narrator is 'unreliable.' Emily Bronte has used 'unreliable' first person narrators to deliberately mystify the shocking incidents in the novel: Catherine's diary entries in Ch3 are suggestive of incest, "we made ... dresser."