HI,
1. When you import a new video into iMovie, it goes into the iMovie library, in a folder titled Original Media, not an Event folder. You can see the Original Media folder by selecting any clip in the timeline of a project, or in the media Browser, and then doing a File/Reveal in finder. A pop-up will show the Original Media folder with your clip highlighted. Also, if you Control-click on the iMovie Library folder located in the Movies folder in Finder, and click on Show Package Contents, you will see a list of your projects. The projects cannot be played there, but if you click on one you will see its Original Media folder in the folder that opens, and its Original Media folder contains the media that is used in that project. DO NOT change, move, rename or otherwise tamper with any of the files and folders in the Show Package Contents folder or any of its contents. Doing so could corrupt your iMovie library.
2. Yes, the empty project will be displayed in the project window as soon as you close it.
3. Once you have finished editing a project, you can share it out form your project window (File/Share, or use the little share button in the upper right hand corner of your project window) to any number of options that you will see listed. So, for example, you can share your project to File, and designate its location on your desktop. Sharing to theater is another option, but you have to choose that option -- the movie won't automatically share there. Also, if you are sharing to File, for example, in the share pop-up box, there will be a little box that you can check to share to Theater, otherwise it won't go there. Finally, in your iMovie preferences, there is an option to share all movies to Theater in iCloud, if you have that activated in your Mac's system preferences.
You can share a project to a movie as many times as you want. The project will remain intact for further editing or sharing until it is deleted.
4. As far as I can tell, the "Events" folders in the Media screen sidebar only show the media for projects that you have deleted, and they bear the same title as the project. The project is not fully deleted until the events file specific to that project is deleted.
-- Rich