The shell was non-root ($).
I've managed to have the reader recognised on the MBA. I did manage to reboot into bootloader as well (the fingerprint was also recognised and worked), but fastboot commands were not processed either. It didn't show the "waiting for device" message, but it simply never got to show any output from
fastboot getvar secure.
I've never used the EDL thing: my Android tinkering days are from the beginning and the middle of the last decade, so I'm a wee bit rusty
If you can link me some documentation on EDL I'll give it a go tomorrow.