Well, it doesn’t look like prototype 4’s motion sensor is going to be working any time soon. I didn’t design the part’s footprint very well, and the pads on the PCB are actually peeling up. Some of them even look like they’ve been burned by the SMD reflow gun. So, without going through all the effort it takes to make a prototype all over again, and on the same version of the board, there’s not really any way to get the on-board motion sensor working.
Fortunately, the particular chip I’m using, the InvenSense MPU-9250, happens to be just popular enough to have breakout-boards, and I’ve found one on Amazon Prime (free one-day, even) for about $12.