The first aspect of your question:
I can only invite you to refer to your company SOP's but as for Airbus SOP's:
"If APU is not available, set EXT PWR to ON, then set ENG MASTERS to OFF" : 3.3.25
About BRAKE ACCU PRESS:
"The ACCU PRESS indication must be in the green band. In case of low accumulator pressure, chocks are required before engine 1 shutdown"
If you lose your BRK ACCU PRESS while Parking Brake is set, different cases:
1/ You have pressure in the Green (ENG running): release Parking BRK et restore braking with pedals
2/ Your Green pressure is at 0: hope that you have chocks in place or that you are on a very flat surface, call for assistance.
This is not applicable to all A320 family A/C as last A/C + modified A/C have a slightly different interaction between PRK BRK handle position and pedals braking (i.e on some MOD A/C, you can use your pedals braking even if parking brake is ON)
So long'