quick & low tech solution:
1. open your portrait-oriented video in quicktime (which is probably an mp4 file).
2. click "save as" in the quicktime "file" menu.
3. create an imovie-compatible video file by using the "format" drop down menu. select anything that will give you a .MOV format file extension. for example, movie, HD 720p, etc.
4. import the newly created .MOV file into imovie.
5. you'll see an unstretched portrait-oriented video in imovie.
hope that's satisfactory.
I also struggled with the same problem for a week (even though my files were .mov already). I found this steps helpful: opening the video clip in quicktime first, then Export it to a certain folder (desktop would be the easiest) using a 720p format (found on the dropdown menu under the "Save As" or "File Name" bar) and then import the video to imovie. This time, the video should not be stretched anymore.
Hope this helps!