First off, leave audio/video cleanup and correction until AFTER you have a final cut. Do all your cutting beforehand, then correct the final version. If you correct all the original footage, you're going to have a HUGE amount of encoding to do. Better to wait until you know which parts of the original footage you're using, then encode those parts. I'm not so familiar with how iMovie does its color corrections and such, but I know standard editing programs let you do this.
Second, when I cut my documentary, I made transcriptions of all the interviews. That means sitting down and listening to what they're saying, and typing it (or writing it, if you prefer). This saved me and my peers lots of time, because instead of going through and listening to each interview over and over, we could read through a script and pick out stuff from there. I'm not sure what the magnitude of your project is, so this may not be neccessary, but if you're going to wade through 3 or more 30+ minute interviews, this is a good idea. Just a suggestion for ya