Continuing with "David Lynch's 10 clues to unlocking this thriller", clue #6 is: Notice the robe, the ashtray, the coffee cup.
Above left: Diane and the hit man in Winkie's. Notice the brown coffee cup. Above right: Diane at home. Note that the coffee cup is the same kind that's used in Winkie's. This is an indication that Diane, at some point, worked in Winkie's. (This doesn't preclude the possibility that she worked at Pink's, as stated earlier in the analysis. Winkie's would have been a 'step up' for her.)
Above left: Diane as 'Betty', wearing a nice pink bathrobe while rehearsing with Rita. Above right: Diane is wearing a worn white robe when her former roommate drops by. The idea here is that the scene with the nice robe depicts a dream, while the scenes in which Diane is wearing the white robe depict reality.
Above left: Diane's former roommate retrieves the 'piano' ashtray. Above right: Shortly after this, Diane experiences a masturbatory fantasy in which she and Camilla are together on the couch. However, the fact that the piano astray is present indicates that we have reverted to a former moment in time, so this scene actually depicts a 'fantasy-flashback'.
The chain of events involving the former roommate are as follows: When Diane first moved into the Sierra Bonita apartments, she moved into number 12. While living there she met a woman named LJ DeRosa who lived in no. 17. At some point Diane moved into no. 17 to live with DeRosa. However, when in Diane's dream, Betty and Rita found 'D. Selwyn' in the phone book, she was listed as living at no. 12, her old address. (Entries in phone books are generally updated only once per year.)
Later, the relationship between Diane and LJ soured, and it was determined that one of them had to move out. Since Diane had already placed the hit on Camilla, and since she knew her therapist had overheard her doing so while in Winkie's, she knew people would be looking for her, so she convinced DeRosa to move into no. 12, and she did so. That's why it's DeRosa's voice that answers the phone when Betty and Rita call Diane's number, and it's also the reason why the two detectives have been looking for Diane at no. 12. When DeRosa comes back to no. 17 to retrieve some items she left there, she awakens Diane, and a brief encounter ensues. When DeRosa says "it's been three weeks", what she means is that it's been three weeks since she moved out of no. 17.
That 'Aunt Ruth' moved out so that Betty could move in, is Diane's dream-representation of DeRosa moving out.
What all this does not imply, of course, is that Rita/Camilla is simply a representation of DeRosa. Camilla is a (real) woman whom Diane placed a hit on, but Diane only fantasized that she had had lesbian encounters with Camilla. The real 'love story' of the movie is the one between Diane and Adam (or some director whom Adam represents). The reason Diane appears jealous and upset when Adam kisses Camilla, is because she's in love with Adam.
[Update 11/13/11: On thinking things over some more after posting the above, I've made the following determinations:
1) There is no actual lesbian relationship or lesbian sex being depicted in the movie. The sex between Betty and Rita takes place in a dream, and Diane's sexual encounters with Camilla are fantasies, as mentioned above; and, Diane only hoped to have sex with DeRosa. What must have happened was that Diane originally moved in with DeRosa when the two women became friends, so that both could save some money on rent. Diane slept on the couch, and DeRosa continued to sleep in the bedroom. Then at some point while the two women were living together, Diane made one or more sexual advances toward DeRosa and was rejected, and this created an uncomfortable situation, so DeRosa moved out of apartment no. 17 as described above.
2) What's going on with the detectives is that they made their first visit to apartment no. 12, since this was Diane's 'address of record', and found DeRosa living there. This visit occurred before Diane went into her lengthy sleep, but DeRosa must have told the detectives Diane was living in no. 12; then, they went to no. 12, but Diane was out.
Next, the detectives started 'staking out' Sierra Bonita in hopes of intercepting Diane who, once she found out about the detectives from DeRosa, 'holed up' in her apartment, at various points being asleep alternating with simply not answering the door. Since by this time she was no longer working, she didn't need to go out very often, and when she did go out, she was able to elude the detectives by exiting and entering via the back - this is what is being represented in the scene in which she and Rita 'duck down' in the cab they're in when they visit the apartments. (Note that Diane doesn't own a car - the detectives have been sitting on the street out front of the apartments, being unaware of the back entrance). Finally, the detectives, being unaware that Diane was at home all this time, went to see DeRosa a second time. Shortly after this, DeRosa dropped by Diane's apartment to retrieve the rest of her belongings. This visit is what awakened Diane, and it was at this visit that DeRosa informed Diane that the two detectives had come looking for her again. The detectives then came by on that fateful evening that Diane was sitting in her apartment staring at the blue key.]
