Yes iMovie 6 should keep everything, even after the trash is emptied. The trash will only reduce project size if a clip is moved to the trash and nothing from that clip is used in the project timeline. If any piece of a clip is used, the whole clip is retained and not sent to the trash.
I haven't dug around in an iMovie 6 project lately, but basically, an iMovie 6 project is a package, meaning it is really a folder containing other (hidden) folders. To see the insides of a project, right-click on the project in the Finder and choose "Show Package Contents."
Be careful within the package, because you can damage your movie and make it unplayable if you don't know what you are doing.
But in general, you will see an .iMovieProj file and several folders. This .imovieproj file will contain internal references to the QuickTIme clips in the Media folder.
The Media Folder in the package contains the QuickTime movies, graphics, and sound files you used in the movie. IF you want to make a copy of a clip, it is most likely to be in here.
The Shared Movies folder contains things like a reference movie for iDVD, a movie to be sent to GarageBand, and a copy of any movies that you have shared.
The Cache folder contains reference movies, plists, and thumbnails for iMovie's internal use. Nothing for you there.
The Audio Waveforms folder stores graphics for the Audio Waveforms if you use this feature.