Eventually broke down and purchased iMovie from the App Store ($14.99). It was not installed on my pre-owned MacBook Pro. I got really tired really fast of trying to figure out whether or not it was possible to do what I wanted to do with avidemux. I’m not 25 anymore; I don’t have time for that shit.
I imagined travelling back in time to 1988 and telling people that in the future they’d be able to purchase video editing software for 50 cents (inflation adjustment approximate) and have it delivered and installed magically onto their computer within minutes (no floppy disks!). This story made me feel better about dropping 15 bucks on something that comes pre-installed for nothing on new Macs.