Follow-up:
Rich839 your tip not only worked, but the knowledge-base it came from - in this case at least - was better than the Apple Store 'geniuses'. I was in NYC for several weeks and pestered them on 5 or 6 separate visits, and got no helpful suggestions. Mostly 'reinstall a fresh copy', which I did to no avail.
In the 10 days since your suggestion, I have been able to merge-edit-export 2 hrs of very satisfying instructional video and am very appreciative.
As an aside, I have been struck by how much having the right tool [in this case iMovie] can expand one's awareness and creativity in doing something. Accessible editorial capability has opened my thinking about both the content and presentation of my subject matter.
FUN!!
Thanks again. John