Re: Why no Mandarin
I think in general the Mandarin would be too offbeat a villain for the first movie --- you're already busy setting up the idea of this high tech suit of armor, the corporate landscape, so you need a more Justin Hammer-type villain that plays into these themes.
I could see him working his way into a sequel, however. I think it could probably work to play him off in a similar way to Ra's Al Ghul in Batman Begins, meaning that you wouldn't necessarily deny the more far-fetched aspects of the character, but they won't need to be specifically mentioned either. The rings themselves could benefit from having that sense of mystery surrounding them.