Story analysis: my take! Spoilers.

I agree with you except for Mima killing the photographer. I believe Mima was actually in Rumi's mock-up of Mima's room when the bag of bloody clothing was found.

Mima's mental health started to wobble ever since she changed her profession - add to this the exploding letter, the anonymous calls & fax, the sex scene & nude photos, and above all the simple fact that she was impersonating other people, which was messing up with her reality a little (plus, possibly, the fatigue - which is why it happens several times that Mima appears to be living her normal life, and when everything starts feeling eerie it turns out that she was on the set). But, in retrospect, she was never quite as crazy as she seemed to us during the movie - which is why she could easily spot the fact that she was not in her room in the end, and that the ghost was actually Rumi (though she still had troubles seeing Rumi instead of the ghost).

In retrospect, we realise that a lot of the weirdness happening to Mina was really happening, it was not all in her mind, and her increasingly spaced out look was a normal reaction to it all. The weirdness was basically a mixture of Mima's personal demons (anguish and traces of regret over the career change, fatigue, awkwardness about the indecent things she had to go through, which is why she keeps having hallucinations of her pop idol self and of the security guard, nightmares and some blurring between her reality and the movie she was filming), the guard's obsession (hence the attack on the juvenile delinquent who had bothered Mima's last concert, the fax & phone call, the mails Mima kept reading about on the "Mima's Room" page, the 3-D visualisation of his conversation with Rumi's impersonation of Mima, the stalking and the attack on Mima herself) and Rumi's split personality disorder; two of the ghost idol Mimas seen by our heroine were actually Rumi's physical impersonations: in the end, during the final fight and chase, and when Mima sees the ghost in a car passing by. Unless I'm wrong, that was actually Rumi in Mima outfit going to CHAM's concert - because somebody was indeed there on the stage, given the awkward looks on the two CHAM girls and the laughter from the crowd, and unless it was Rumi herself, their agent, the girls would probably have stopped singing. Further weirdness issued by Rumi were, of course, the murders (all of them, they were done by the same person, and it was somebody who knew who had written the movie, who had taken pictures of naked Mima and who will make yet another movie with sex scenes for Mima), the internet site, the fake Mima room, and the bag with blood-stained clothes placed in Mima's closet.

Indeed, Mima and Rumi were both mentally troubled, but while Mima's problem was transitory and issued mostly out of stress and tiredness (...and pranks played by Rumi), Rumi was clinically ill - double personality, one of which murderous.

there's a highway that is curling up like smoke above his shoulder

It makes sense what you say, but I think anything is posible in a MPD story. There is no just one correct explanation.

If you like this subject, I would really recommend you to see Identity (2003).