I'd interpret the datasheet that way that this device is SPI mode 1.
const int CLOCK_SPEED = 10000; // DAC is rated for 30MHz, Arduino clock is much lower? Signal started looking really bad @100kHz
The slowest possible clock speed is 125kHz (16MHz / 128), the hardware cannot make it slower.
The hardware is allowed to change the MOSI line in mode 1 and 2 as long as the signal is table at the falling edge of SCL.. The resolution of the screen shot is to low to tell if the timing is correct but it seems that everything is OK. This is not bad practice, is simply inside the specs.