eMMC support is provided transparently. Just flash the image to the eMMC board as you would an SD card.
By default, console access is granted over the serial header and over HDMI. Certain startup messages will only appear on the serial console by default. To show the messages on the HDMI console instead, swap the order of the two consoles in the cmdline.txt file on the boot partition. You can also delete the AML0 console if you don't plan on using the serial adapter.
eg. console=ttyAML0,115200n8 console=tty0
Refer to the odroid wiki. At this point not all functionality is supported by the upstream kernel used by Home Assistant OS.
The GPIO on pin 11 is used as a low active power button input.