Talking to some of the folks on the film, the script that Neill went into shooting D9 with was pretty different from the final cut of the film, so I think Neill and Terry decided they didn't want to let one go out into the wilds. Plus, yes, a lot of dialogue was improvised as they shot...

Yes
District 9 is impossible to find. Ive searched high and lo as well and am still unsuccessful. Ive asked some friends and they said that is pretty hard to find do to the fact that D9 wasn't big till it premiered really. That being said, it is hard to find. This is what Ive been told. Although I had an interview with Jason Cope earlier this week, I probably should've asked. IF you'd like to read the interview, the site is below.

movieman49.blogspot.com

Nothing on District 9, and there used to be a copy of a second (maybe third?) draft of Iron Man in .pdf form floating around a while ago--it was from pdfscreenplays, though, and you can't get to their scripts anymore unless you're a subscriber I don't think.

Here's the original post: http://www.simplyscripts.com/2008/05/12/iron-man-script-no-not-the-one-in-theatres-and-go-speed-racer-go/

Clicking the link just brings you to pdfscreenplays homepage, though :/. Someone has to have grabbed that at some point, though, so it's out there.