One way to determine what kind of files you have is to start a terminal window (under applications/utilities) and use "file".
Once you start the Terminal app, you will get a prompt string ending in "$". The $ is shown in most documentation to remind you that you're at the command prompt, but you do not type it, it's provided.
In the terminal window, use the "cd" (change directory) command to navigate to the old movies. For example you might use something like
$ cd /Volumes/MacOS8/robin/Movies/iMovie\ Projects/My\ Movie
Then use the confusingly-named "file" command to identify the file types
$ file Media/*
Media/Clip 01: DIF (DV) movie file (NTSC)
Media/Clip 02: DIF (DV) movie file (NTSC)
Media/Clip 03: DIF (DV) movie file (NTSC)
Media/Clip 04: DIF (DV) movie file (NTSC)
Media/Clip 05: DIF (DV) movie file (NTSC)
Media/Clip 06: DIF (DV) movie file (NTSC)
Media/Flying Words 01: DIF (DV) movie file (NTSC)
Media/Flying Words 02: DIF (DV) movie file (NTSC)
$
Now you see that they are DV movies; that is the file format you get when iMovie imported form a video camera.
These files can be viewed in the free VLC app, and can be opened in Final Cut Express (I don't have FC Pro so I can't speak for that). But as long they show up as DIF/DV movies in "file", they are probably intact and you will eventually find software that can process them.
I just re-created a movie from 2002 in FC Express using this technique. Just do File->Import Media and open all the DV files, drag them into correct order. You might lose transitions and the like, but all your important video and audio should come through.
In fact, you can also convince iMovie (10.1.4) to import these files. Rename the files from e.g., "Clip 01" to "Clip 01.dv". Then in iMovie use File->Import Media and select the files; they will now open. Again, I don't think you can get all your transitions etc., but at least you can get your audio and video. And titles, if they were rendered.
Hope it helps someone.