Walnut
I note that the a/c needs 2 GPUs to provide engine starting, I have been to numerous outstations where you are lucky to get even one doubtful unit.
I believe this is one reason that the APU can be started by either battery or ground power. The APU can then start the engines, one at a time.
If there is no fuel on board for the APU, the main battery will power the fuel transfer system to allow fuel to be added and made available to the APU.
... but again the pilot is faced with pressurisation problems...
I also believe that since pressurization is provided by electrically powered compressors rather than air bleed, minimal cabin pressurization can be powered from the APU or even just the RAT.