Short answer: Triggertrap app audio triggering latency is too long to capture a fast moving event.
The app, the dongle, my trusty EOS Rebel XTi, Lensbaby (manual focus, soft edge details), and Neewer flash worked, but too slowly. The phone was just inches from where I was throwing the dice, but the flash and camera were triggered after most of the action happened. Most of the time the die flew off the table before the picture was captured. The delay was such that I was able to slam the die to the table and remove my hand from the scene without being captured on camera.
On the other hand, the Fuji Instax tips and tricks have proved handy over the past couple weeks.
I’ve gotten a bit better at managing focus and I can definitely confirm the camera doesn’t perform in the dark (heck, this was even a good blue hour shot that it couldn’t pull off)*. I still struggle with it though, and rarely get the focus, exposure, and framing exactly to my liking. It’s still great fun, just challenging.
* Yeah, the lesson here is that I should try earlier, during the golden hour, or bring lots of flash. Alternatively, a Belair with instant back and a longer exposure might have done it.