Here are my thoughts, hope you appreciate it and would like feedback where possible:

I believe that the person in the beginning falling asleep is dianne as aunt ruth(in the apartment) dreamt by dianne hence making mulholland drive a dream or reality within a current dream, and dianne as aunt ruth again when she hear's something drop(the blue box) hence being 'woken' up out of mulholland drive, coming back to this current dream in the villa(although we visually see aunt ruth(dianne) come out of a room and to our eye couldn't possibly asleep(the deception)etc, then from this current dream that was in the beginning, the cowboy wakes her up.

***I believe aunt ruth is actually the clairvoyent lady because for one it would make sense because aunt ruth is dead, and psychics conventionally known to be links to the other side etc.
But what also reasserts this is when the psychic goes to coco "i want that one out of my house"(referring to dianne's neighbour whom swapped apartments with dianne). See, aunt ruth's or diane's dream-like apartment is a hollywood villa which is also imagined and placed over the top of dianne's real apartment (reflected in the morphing transition shot where the cowboy wakes dianne up from the dream).

**Mr O'rourke and everyone(the henchmen aswell), if you will, is dianne's iron fist. It was represented at certain levels, shown where they were dialling to each other (stating the girl was missing)starting from the elite Mr Orourke, to the fat guy with the chandaleer(can't spell), down to the fat arm of a man in a dirty place(resembles dianne's real apartment)whom calls a black phone which goes unanswered, because dianne turned into betty(the next shot is betty arriving at the air port, hence the phone being unanswered). We can also say now that the second part of mulholland drive is a fradulent dream-state but with more truth(but more of a nightmare), aswell since dianne answers that black phone near the red lamp- notice also how the there is a silhoette cast upon the white wall which is the outline of shapes for dianne's real apartment door.

***The whole thing about Mr O'rourke and their mission represents the whole murder which was in real life, their dire need to cast the girl is the absolution of the event and it had to satisfy what happened in real life. The casting of the girl "this is the girl" is simply a metaphor for the murder, the Lead role in a block buster movie is a clever metaphor the murder,she subconsciously replaces camilla with the other actress (Because of her resentment as shown in the 2nd MD sequence) so camilla can exist as rita and hopefully fall in love in a romantic way with betty, this also suggests that both MD sequences are related and are not subject to time and maybe not reality at times. The second MD sequence is more of a nightmare and more confronting.

Rita's amnesia and the subsequent events represents the clean-slate love that they could've had which dianne consciously contrives for her own ideal world.

**I can't explain everything, but i 'feel' what the story was about because rarely do we get any insight, visually that is of real-life events, as to what happened really happened.

The good thing about the movie is that it is supposed to be 'felt'and ultimately understood with intuition(and some information)without hand feeding us the story. I think people are making too many contrived attempts to dissect it-where it should just be understood like any painting. People are perceiving things too literally which ultimately just satisfies their own curiosity for who is who and what was dianne's background. I think it's unnecessary to do this, however there are certain suggested facts which most of us agree with.
The story and overall emotional overtones are presented chronically in a dream-state face-value story, but this does not mean that we are presented with the real-life events chronilocally.
It's pointless to argue whom certain people are with people like the old couple, where many people have said are dianne's grandparents etc. They could be, but what substantiate's this? Because they're old and they have names and are accommodating to dianne in the beginning? I don't even think they're warm or anything, they make me edgy and scared- and i get a unsettling sinister vibe from them. The only sense we feel is that they are evil in every frame they're in- ie. the wicked smiles and laughter in the limo and of course the end. David lynch is an artist and a film director, he told them to look evilly elated while laughing and he chose these symbols to look like that.
This extreme evil is further conveyed metaphorically in characters such as the old couple, the "mud woman"(as everyone describes her as), mr o'rourke and the cowboy.
The symbolism conveyed in objects, characters, monsters, lighting, dialogue, music, anything really etc provides us with the right instincts as to what this movie was about. We can only suggest and speculate whom was in dianne's, adam's and camilla's life by the clues/charcters which are thrown into the journey.

The only visually literal insights of what 'real life' was, was when we see dianne wake up in her Real apartment making coffee, dianne's recollection of the sylvia north story, masturbating, dianne talking to the hitman, and finally dianne killing herself, that's all. The dinner party is not real, it's another mulholland drive fantasy which went wrong but may have been based on an actual event, or not- it's just like the the first MD fantasy, ie. dianne conjure's up a romantic mystery where camilla is hiding from her persuers and dianne is her saviour, and finally getting romantically involved(which to dianne's delight, which highlight's dianne's needy, emotionally unstable state, ideallistically created as a result of jealousy ,unrequited love and murder).

The two mulholland drive journeys are both products of past events and of each other.
Mulholland drive or the 'event' of mulholland drive is dianne's subconsciousness' idealistic fantasy of both making it in hollywood and acquiring the romantic love of Camilla(Rita), but i think more so her relationship with camilla.
I think attaining camilla's love was more important than making it in hollywood, but this too is pointless arguing about as both went hand in hand with dianne's self destruction and fantasies we know as mulholland drive.

The dinner party was real. It was one of the first "real life" scenes in the movie. The humiliation Diane suffers there precipitates the "murder for hire" of Camilla. The money Diane hands Joe the hit man in the diner is the same money Rita (Camilla) has in her purse at the beginning. The first 2/3 of the movie is a dream that Diane is having driven by her guilt. The cowboy wakes her up and she spends that morning thinking about what she has done, how her career is in shambles, how Camilla rejected her and the finality that Camilla is dead. That is why she shoots herself. The last part is real life, but with Diane's psychotic imagery.