iMovie-multiple questions/concerns

Hi all! To start, I currently work on a MacBook Pro, v10.13, High Sierra. (FYI, for other reasons, I cannot/do not want to upgrade any higher due to using standalone versions of Adobe’s PS-CS6 and LR5). I stupidly opened a forum question elsewhere re: 2 separate issues, when it became apparent I should have kept my iMovies issue as a different topic. Before I begin this new question in the iMovie Forum, and so as not to create any redundancy, I’m sharing the discussion from the other incorrect forum. ALSO, PLS NOTE I have noted pertinent concerns I still have with a (**) - I could still use further clarification and/or answers. 

See below screen print. When I did an “About This Mac > Storage > Manage > Recommendations, in order to “Reduce Clutter”, all these movies are sitting here, but when I tried to do some research on my own, some online feedback about what to delete in iMovies on the Mac is confusing at best (i.e. readers we’re being told to delete the “Events” and not the “Projects”). Not sure this s/b done - (**) what if I want to go back and edit a previously-finished movie and resend the newer version; will I still have that movie to work on?

This from MartinR (in the another forum):

Regarding iMovie, your iMovie Library contains all your iMovie Events, Projects and editing. In your case your iMovie Library is in its default location in your Documents folder (and can be seen in your screenshot).

Each Event in iMovie is associated with a movie clip (a file) that resides on disk. If you delete any of the clip files that are represented as Events in iMovie you will no longer be able to re-edit because the clips you deleted from disk are gone. So, you'll have to figure out what you want or need to keep before deleting any iMovie Events or their associated clip files on disk.

When you export a finished movie from iMovie you create a new file on disk of your completed movie. That finished movie is separate from the clip files.

-Read Difference between Events and Projects for a bit more info.

-There is also an iMovie forum here where you can explore and get help with iMovie.

My reply to Martin: 

Correct me if I’m wrong, but (**) if I have saved these files to other separate external HD’s (I typically back up any photos and videos to more than one ext HD), then I can safely delete these from my Mac's Documents, right?? I mean just the Quicktime movies, NOT the iMovie Library? 

From MartinR:

If you have copies of all the the source media files (video & audio clips & photos) on backup drives then yes you could delete the copies in your Documents folder.  But first make sure about what you are deleting so you don't inadvertently delete the only copy of something.

The only issue there would be later ... to the extent that your iMovie projects were built using the source media files in /Documents, once you delete them from that location they would go offline in iMovie. Later, if you re-open an iMovie project that used those files, in iMovie you would have to reconnect (aka relocate, relink) to copies on whatever drive they are on at that point. (** - need some guidance here; don’t know what this entails).

My reply:

Ah, thank you!  Understood. Instead of whacking them all at once, I'll test one at a time.  Again, thanks sincerely for your knowledge and advice.

My reply next day to MartinR:

Thank you for your response. I read the link re: differences between Events & Projects, but when I look at my iMovies Events and Projects FOLDERS separately, they don’t “sync up”.  (**) Is it possible to have Events but no Projects and/or Projects but no Events??  (**) If I am creating a video combining both video clips from my camera as well as single photos, does this make it BOTH an Event and a Project?  (**) When the word “clips” is used, does it only relate to VIDEOS I’ve recorded (and NOT photos)? Re

From MartinR:

It will be better to pose those questions in the iMovie forum.

So, here I am… Sorry so long-winded. TO RECAP, basically I want to reduce storage on my Mac. These movies (in the screen print, in my Documents) take up a lot of space. Copies of these Quicktime movies are contained in separate External Hard Drives. The iMovie program remains on my Mac. Can’t I deleted these movies in my Documents and if I wish to work on or edit the movie further, copy them back to my Documents? If not, why? What am I missing? (And, yes, in the other forum, I have addressed a major issue with SYSTEM storage taking up so much space - per bottom left of screen print).