Oh, I agree. I'm sure Murakami put the least effort possible into managing Eiri's mental problems. I was basing my opinion solely on Eiri's personality in the manga, and I'll agree that even that leaves a looooot of room for interpretation. Even with just that, though, I think it's a pretty safe bet to rule out MPD for Eiri (and probably Ryuichi, too, although we're given even less to work with so far as he is concerned).

So far as the woman you know goes, I'm not an expert on MPD, so I don't know how common that is or not. I know that, typically, MPD patients don't know of their other personalities unless they've seen doctors or on medication.

*gigglefit* In other news. I'm on the phone with Kyle, who is being an asshat, and I asked him what he thought about the Eiri-disorder thing. The following is a paraphrased conversation:

Me: So. Eiri and his possible mental disability. What do you think about it?
Kyle: Mental what-ness?
Me: ...
Kyle: Oh. Sorry. I'm tired. Eiri and his mental decapitation, hm?
Me: ...disability
Kyle: Right.
Me: ...so?
Kyle: Do I still get to fuck with his character as much as I want?
Me: Um. I... guess?
Kyle: Then I don't see why I should give a shit. I'll stick him on whatever medications pop into my head and he can have whatever disorders correspond to those medications. Oh. And he's going to drink tea. Lots of tea.