"How would a mirrored boot work in practice?" The same way it has worked for the past 30+ years: the firmware boots from first designated boot device; if that

"How would a mirrored boot work in practice?" The same way it has worked for the past 30+ years: the firmware boots from first designated boot device; if that device isn't bootable, it moves on to the next one. The next one boots since it's part of a mirror and has all the necessary data to do so, and in the rare case where the device is "half bootable" one would simply intervene and select the next good bootable device manually.