Similar issue, but not as elaborate testing! I started a movie with a PIP at the beginning. Guarantee crash if I try to finalize, export, or play. I gave the pip a 1 second delay at the start of the movie and it worked fine. Still a bug, in my opinion. Also, imbedding a transparent logo, use png format and match the aspect ratio of the image to the movie.
After many, many, many MORE hours of testing and dragging my 27" iMac into the Apple store (twice), the decision was "I don't know what's wrong". We ruled out PIP, iMovie itself, user account, all my photos and videos, and probably some more things that I have forgotten. We decided to reinstall the OS.
Turns out, you can do a clean reinstall by reformatting the hard drive, etc, very painful. Or, you can just reinstall on top of the existing configuration. We chose the latter, holding out the most painful procedure for the really really last resort.
It worked. I can now finalize my movie. The only problem is that it's a year old and I no longer care enough about it to watch it. I'll probably get over that soon. Everyone else in the family likes to watch it.
I hope this helps.