So, what protects the APU if it catched fire during an auto-shutdown, with no personnel present? (Must emphasise this is not a 787-specific question.)
Usually the APUC stays powered for approx 2 mins after the APU shuts down. I think the 737-3/5/500 had problems with this early on as pilots/engs were shutting the battery off as soon as the APU started to shutdown. The consequence was that on the next APU start a failure would occur as the APUC was still in shutdown mode.
Back to the 787.
If Boeing insist on a battery that 'may' fail but it's 'safe' as it will be contained then the APU needs a seperate power source. IE A PMG that will continue to power the APUC/FADEC when the APU is up and running.
If the backup can't be relied upon then the backup needs a backup.
What a mess!