i realize you solved this already and got your system working like you wanted, but i was reading through your solution after you posted a link in the efivars thread and thought i would share a link as well.
a couple of weeks ago someone was having an issue installing mint to an external drive. since i didn’t really understand your solution at the time, i didn’t see the link between the two. i tried a quick mint install to a usb and sure enough, the installer ignored my direction and made a mess of grub on my internal drive.
after a bit of discussion, someone posted a helpful link that showed there is indeed a bug in the ubuntu/mint installer dealing with uefi and external bootloader installs specifically.