All of this information is very helpful. Here is what you need to try.
Look at the file aml_autoscript in your armbian /boot folder. This is the script that should get run when the reset button is pressed during poweron on a regular amlogic based android tv box.
But since you now have access to the uboot console, you can enter these environment commands directly.
If you start with the 'setenv bootcmd...
and end with the 'saveenv'
you will have modified the uboot environment to run armbian from sd or usb first, and then fall back to your other os on emmc.
In looking at your current uboot environment, the only command that will be modified from what you have is the 'bootcmd'. In looking at what you have both the armbian and your current bootcmd fall back to run 'storeboot'. But by installing the armbian version you will loose the abiltiy to boot osmc from sd or usb, which you probably don't care about, but I thought I would mention it.