It looks like you have some corruption either in the iMovie Library or the iMovie app. You'll need to do some troubleshooting.
First, try opening iMovie while holding down the Option and Command keys, and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. See if that is any help.
If no luck, open iMovie in a new or different library. To open in a new or different library open iMovie while holding down just the Option key. A box will appear on your screen listing all libraries that you have. While the box is on your screen see how many libraries are listed there and open them to see what is in them. Sometimes projects inadvertently get saved in different libraries than you think. If your projects are not showing up in any other libraries, then create a new library by selecting the "new" option in the box, and do a small test project to see if the project saves correctly. If it saves correctly that would indicate some corruption in your old library.
Failing the above, try doing a reinstall of iMovie 10.1.9. To do that, put your current iMovie 10.1.9 into the trash, but don't empty the trash. Then redownload iMovie 10.1.9 from the app store. Once the reinstall has been successfully completed, you can empty the trash of the older iMovie.
If still no luck, then you will need to try to restore the old library. You can do a restore from your Time Machine backup if you have one, at a point in time before you believe that the corruption might have occurred.
If you don't have a time machine backup, or other external drive backup, then you can look in the iMovie Backups folder on your Mac. To get there, start at your desktop and follow this file path: G/Home/Library/Containers/com.apple.iMovieApp/Data/Library/Caches/iMovie Backups. When you get there you should see a list of previously backed up iMovie libraries. Click on one dated before you believe that the corruption occured. iMovie will open in that library. Check to see if your projects are there.
Regarding importing a previously shared out movie back into an iMovie project, it will import as one large video clip. The edits previously made will have been merged and embedded into the video. As you have found, you will not be able to make more than limited changes. For example, you can split the clip, delete or move the splits, overlay different titles, insert different transitions between splits, and so on. You will not be able to undo or change any previously made edits.
-- Rich
