Managing two iTunes users on one computer - a complicated scenario.

Question

Here's my situation: I have two users/accounts on my computer. Each user wants access to all the media (music, podcasts, etc) on the computer.
However, there are a few specific constraints:
1) Each user's instance of iTunes should only display the media they want it to display (for example, if they don't want a certain song, they can remove it from their iTunes library, but it would still exist for the other user).
2) Star ratings, play counts, and other sort-of meta data must be unique for each user (for example, if user one assigns 5 stars to a particular song, that won't affect user two who only gave it 1 star).
3) If one user deletes media from their library it won't suddenly disappear from the other's library because the file has been deleted.
4) No duplicate copies of any given media file (for example, if both users like a particular song, there would not be two actual copies of that song's file laying around - each user's library would reference the same file).
5) No unused media files (for example, if neither user wants a particular song, the actual file would be deleted and would not just sit there unused eating up space).
I hope that makes sense. Any ideas?

Answer 1

I'm not worried about downloads from the iTunes music store (yet)... I'm primarily concerned with existing music. I tried following Apple's instructions for sharing music:
a) I moved both account's media folders to a shared location.
b) Then, using "Add to library" I added the contents of each media folder to the other's iTunes Library. At this point both users had access to all music, and star ratings/play counts were unique for each user.
c) If user A didn't like a song from user B's collection, and removed it from the library, it still appeared in user B's library. And vice versa. Great!
d) If user A didn't like a song from his own collection, and removed it from the library, it still appeared in user B's collection so long as user A didn't also trash the actual file. Great!
That left me with two problems: First, duplicate copies of song files. This wasn't a big deal because I could slowly weed them out - but still kinda annoying. And second, unused (by either user) copies of song still had their files there taking up space. This was problematic as I couldn't figure out a way to find these files and trash them.
I figured surely there was a better solution - be it how to handle the above problem, or a completely different method of setting things up to begin with.