I am new to FreeBSD, so please forgive me.
The path to the device is a bit odd, I am successfully able to unlock GELI encrypted devices where the device files are directly under /dev. I suppose you have encryption on top of a mirrored device? I haven't come across that yet to know what the path / device structure looks like.
Otherwise, your configuration looks right if the devices were present @ /dev/mfid0p8 and /dev/mirror and similarly, your keys were under /root/{1,2}.key.
I did search for this:
https://wiki.freebsd.org/MasonLoringBliss/UEFIandZFSandGELIbyHAND
Perhaps those tidbits might help.