Rita is not real. She is the main part in Adam's film that both Diane (Naomi Watts) and Camilla (Melissa George) want. Diane sees "Rita" as named "Camilla" at the end because Camilla got the part.
Rita is not real. Think: Sixth Sense. Then think of some of the scenes in MD as Adam's film set. Rita is an hallucination. She is "the part" come to life for Diane. Diane's scenes with Rita are psychotic manifestations of the part in Adam's film. Rita is not "Camilla." Melissa George is "the girl" Camilla. Camilla got the part of Rita.
Betty is her real name and Diane is her stage name.
This is a film being made within a film. "Betty" (Naomi Watts) is an actress who hopes to be hired to play the role in a film of a character who is named "Rita." Rita (in Adam's film) was a criminal who helped in a heist and was in a car accident on the highway when they stopped the car and her accomplices forced her at gunpoint to get out so they could take all the money.
Adam Kesher is the director of the film being made about "Rita." "Betty" auditions for the role of "Rita" and wants to play the "part" of "Rita" in a film about the woman who was part of a heist and is in a car accident. The amnesia is Betty's loss of identity into the part she wanted.
The beginning scene with "Coco" was part of Adam's film. Adam cast his mother as "Coco." The detectives at the beginning are actors in Adam's film. At the end, when you see "Diane" in the identical scene in the car being told to get out of the car, it is based in Diane's head on a real scene from the film. It is the part Betty wanted. Maybe "Rita" is killed in the film version when the gangsters catch up to her, and the "body" Betty and Rita "discover" is part of Adam's movie. It is a scene from Adam's script.
Also, when "Rita" wears the blonde wig... it is because "Betty" is getting her own identity mixed up with the part she auditioned for. She "sees" Rita in the car with the director when he is demonstrating the kissing scene, but wishes she was in the car being told how to play the scene. She is not jealous of Adam kissing Rita. She is unraveling because she did not get the part. She never got the part because the other girl, Melissa George as "Camilla" did... the other girl was "the girl." "Camilla," the actress, is playing "Rita." There is no "Rita." "Camilla" is Melissa George who Diane sees as "Rita" because she got the part. Aunt Ruth doesn't see Rita because there is no Rita. It was a part in the film.
"Betty" has everybody mixed up in her mind because she has totally confused the characters (in that film she wanted to be in) with the real actors cast in the film and the parts in the film and her own identity. And "Betty" loses her mind when she doesn't get the part, when she is not "the girl," and her life takes a different turn when she is not successful in Hollywood and she falls in with those criminals. Her hopes are gone. She sees the "keys" to her future and imagines the "wizards" control her fate. Club Silencio shows how reality is not all as it appears and the club is Diane's subconscious letting her know she is losing touch with reality.
Betty/Diane is not wrapped too tight and she confuses the part she almost got ("Rita") with her own identity. Remember at the beginning she tries to assign the amnesiac character in the film, "Rita," the name "Diane Selwyn," and they look through the phone book? There is no "Rita." It is all an hallucination and Betty unraveling. Diane wants to be "Rita," the part in Adam's film.
She kills herself at the end because she has completely lost her own identity and her mind and can no longer separate truth from fantasy. And her dreams are gone. She kills herself just like the character in the film she wanted. She dies the same way. The old people at the beginning were there when she was filled with hope and at the end they are there to kill her dreams.
P.S. The clues are right at iMDb in the credits. "Rita" is Laura Harring. Rita doesn't even exist. She is not real. She is a part in Adam Kesher's script. Melissa George is Camilla, who gets that part.

