Yes, Director "apps" themselves are totally cross-platform, but as you suspected, the Xtras are targeted for specific OSes. If it wasn't for the Xtras, porting would just be a matter of testing on both platforms to shake out any platform-specific Director bugs.

As far as the best way to get to the Lingo source, I don't know. My Director-fu used to be pretty good, but that was 7 years ago.