Time passes 
I decided to buy a serial to USB cable:
In /etc/default/grub.d, I changed 50-cloudimg-settings.cfg and 60-libre-computer-settings.cfg so they both included the line GRUB_TERMINAL=console. Changing the grub timeout in these files seemed to have no effect.
I connected my serial cable to TX2 and RX2 on the SBC per LoveRPi’s instructions on the Amazon page: RED → 5V POWER (Optional) GREEN → 3.3V TX to RX OF DEVICE WHITE → 3.3V RX to TX OF DEVICE BLACK → 0V GND
I booted the ROC-RK3368-CC (SBC), then plugged the USB end of the cable into my Windows laptop and ran Putty with a serial connection, baud rate 1500000. I went to device manager in Windows to see what serial port the LoveRPi device installed, it was COM4 for me.
I ran “sudo reboot” and lo, I saw the grub menu in my Putty terminal! Quickly pressing the down arrow stopped the menu.
I tried
boot_target=mmc1
boot
but received an error about needing to load the kernel first (?).
I did some more research and found this grub menu hack, which I applied:
The next reboot, I selected the new grub menu entry I created per the link above, and lo, I booted into onto the MicroSD instead of EMMC! Woo hoo!