Once I had the two video recordings in hand, or rather on disk, I looked around to see which video editing software available for OSX on the Mac would be able to help me. This is nothing new to me; I've used video editors in the past. I did discover, though, that there were new options available. I decided to give OpenShot a shot.
As it turned out, OpenShot was perfect for what I wanted. I could put each video recording on separate tracks. By making the first track semi-transparent (alpha=0.50), I was able to see both videos overlaid which allowed me to synchronize them properly. I muted the lower-quality audio track, the iPhone recording in this case. Then I used that semi-transparent to see when were viable times for transitioning between the two different points-of-view. The main editing I needed were all available menu options: slicing videos, fade-in, and fade out. The only difficult part was the time, since effectively I had to watch the video in real-time, frequently re-playing sections to find the right cut points.
Once the point-of-view transitions were set, I explored adding a title. OpenShot already has great title sequence options including animated titles generated by Blender. This turned out to be fast and easy, and I love how it turned out.
This left just one question, whether or not I could incorporate my slide presentation effectively. I discovered Keynote has an Export-to-Video option, exactly what I needed. Once I renamed the .m4w file to .mp4, OpenShot took in the movie file no problem. A brief web search led me to the answer I needed: by just editing the clip's properties, I could put the slide presentation video over a small portion of the entire video. In this case, I just laid it over the screen in the video where I was showing the presentation in real-time. Then it was just a matter of slicing up the slide presentation videos so the animations appeared at the right times and timing the slices and the fade-ins and fade-outs so that I was not obscuring myself.
It took some time, as all new things do, but the end result for a first-time experiment was much better than I had expected. I could not upload the full-quality output to YouTube because it was too large, 3.65 GB for a 30-minute video at 1920x1080 and 15 Mb/s, but the 5 Mb/s 1280x720 render came to 1.27 GB and still looks pretty good!