The SPI pin locations comply with the BOOST-DRV8711 interface. I have reason to believe that the registers are never being written through the SPI interface:
I attempted to write the enable bit (register 0, bit 0), and I sent pulses on the STEP line. The outputs to the motors remained inactive.
What is the polarity of SCS? I have it coded as active high, according to the datasheet, but it is worth a sanity check.