1.

degree

screenplay

A young unsuccessful actress, an amnesiac seeking identity, a director who lose control, an unknown dead body. BREAK. A waitress who lost her illusions and contracts a hitman. a suicide.

This makes little sense, and requires a follow up (sequel) to understand the narration gaps. But MD contains the answers within itself, so a re-reading will reveal another level of understanding, deeper in the mystery.

2.

degree

Dream

Betty is Diane's fantasized image, likewise Rita is Camilla's fantasized image, in Diane's dreamt up reality. So the movie is a mirror of reality and dream of the same events.

3.

degree

De Rosa 

DeRosa is actually Diane's partner more than was ever Camilla, so Diane transfer her relationship with the "girl next-door" onto a rich glamorous actress. So most of the movie is like if Diane tell us (or herself) a beautiful story, exaggerating reality with emphasis and mythomania. She tells us about her dream.

4.

degree

Dan

Dan is obsessed by a vision of a monster (the censorship of subconscious: expression of a repression, of a terrible truth that cannot be worded) and introduce us to his demons impersonated by (his anima) Diane's life (where all are 1 person: Diane/Betty/Camilla/Rita the multiple aspects of his inner conflicts dealing with theoretical studies of desire/repressed love/wish to kill/social oppression...) We look into Dan's (Diane - I guess someone already noticed that) analysis.

5.

degree

Lynch

Lynch uses this movie to express his fears and disappointment about movie illusions in particular, and about love subconscious feelings in general. He writes to relieve himself, and encrypt it with dream symbols, by a transfer of himself into Dan and Adam.

6.

degree

Jung

All belongs to the collective subconscious that even escapes Lynch's awareness, unintended symbols, Freudian slips, non-rational development, dreamy stuff visions (his springing ideas that feed his work=emotional inspiration), and the decoding of this teach us about each and all of us, because those fears, demons, desire, taboos work the same for everyone.