Hi,
Best practice is never to delete anything from iMovie.
Here is an iMovie online help link that provides more info re deleting events and clips:
https://help.apple.com/imovie/mac/10.1/#/mov74d7d370c
One thing that you don't want to do is to delete media from the iMovie Library Original Media folder, as that will permanently remove the media from the iMovie system. Doing that would delete the media from any project that might be referring to the media, thus essentially disabling the project. You would also lose your original media unless it were backed up elsewhere outside of the iMovie system.
The Project Media is akin to an event in itself, so I don't believe that deleting clips from the original Event would affect any clips that are in the Project Media. However, you should experiment a bit to verify that for yourself.
It can be a little tricky sometimes to determine when deleting an Event or a clip from an Event will remove it from the Original Media folder or impact a project. To be safe, I don't ever delete anything from iMovie.
I tried an experiment whereby I created 5 clips named 1, 2, 3, 4, 5 respectively. Then I created a new iMovie Library and imported the clips into a created Event. Then I created a project and dragged (moved) clips #1 and #2 from the Event into the Project Media, and dragged clip #1 from the Project Media into the project timeline.
So, at this point the Project Media and the Project original media folder contained clips #1 and #2. The Event Original Media folder contained clips #3, #4, and #5, but not clips #1 and #2. I then Option-dragged (copied, not moved) clip #3 from the Event into the Project Media. Clip #3 showed in the Project Media but not in the project's Original Media folder. Clip #3 continued to show in both the Event and in the Event's Original Media folder.
I then deleted the entire event to see how that would impact the clips in the Project. What I found was that the Event was deleted from the iMovie Library. However Clips #1, #2 and #3 remained displayed in the Project Media and in the Project's Original Media folder. The project played fine with clip #1 in the timeline. Clips #4 and #5 were permanently removed from the iMovie library.
I tentatively conclude that one can delete an Event without impacting the project as long as the clips in the event have previously been copied or transferred into the Project Media. Any other clips in the Event that had not been copied or transferred into Project Media were deleted from the iMovie system.
My experiment was not exactly scientific, so don't rely upon my findings but confirm them yourself.
-- Rich