So, it turns out, if you populate the MPL3115A2 upside-down, it shorts out the I2C bus and nothing works.
But I did manage to scrape it off the board. Unfortunately, I damaged the PCB in the process and won’t be able to reinstall the barometer. But I was able to salvage the rest of the board, so I can use this one as a dev test board until I get the firmware completely sorted out. 😀