After some troubleshooting, I concluded that the issue with these projects was not inside the projects themselves, but in the events that have been used in these projects and to be more precise, with the thumbnails of these events.
It looks like something went wrong during or before the iMovie crash and affected the events video thumbnails.
The solution in my case:
I forced iMovie to re-create the thumbnails of these events, by deleting
the thumbs folder inside iMovie events folder in Finder.
Here is some further info, regarding the issue and how I ended up with recreating the thumbs:
The iMovie owner, had a special structure in separating his iMovie projects in folders by project topic. I found out that it was only some 8-10 projects under a specific folder that were unable to open-load inside iMovie. Some of them, they were able to open, but all they were showing in the viewer was just a gray screen.
Also, its worth noting that all the other iMovie projects in the library were working fine.
I also noticed that when right-clicking on the problematic projects inside the project library, I was not getting the extended contextual menu that presents commands like duplicate, export etc, but only 2 commands: to create new project or new folder.
Trashing iMovie preferences file had absolutely no effect, nor renaming the project files either. Searching on Google for similar issues, I couldn't find anything similar. Running onyx's cleaning features, repairing permissions, checking hard-drive system for corruption - all these were also in the mix - but with no result.
Finally, I had an idea:
I sent a project file (containing no events/video) to my own mac and tried to open it. It opened just fine, with the exception that it couldn't find the source files.
That lead me to think that the project files were fine and that I rather have to check for the events.
The owner had a similar folder structure for his events as well. So, all the events used in the above projects, were inside their single folder in iMovie Events folder. As first move, I moved that folder outside of iMovie events folder and opened iMovie.
At this time, all projects were able to open. Also, the extended context menu appeared back. I was close to my final conclusion but yet I wasn't sure what was wrong. The videos, the caches, the thumbs? So, putting back in the events folder, one by one files and folders, I got my answer.
The thumbs were corrupted.
Just to make sure everything will be fine, I finally let only the actual events video files inside that folder, opened iMovie and let it re-generate/optimize the events videos of that folder.
It took about 4 hours for iMovie to complete this, but after that all projects are working fine.