Winding back the EPB motors with a PP3 battery is incredibly simple. I followed this forum guide, which initially describes removal of the motor but then goes on to also suggest the use of a battery. I preferred the battery approach as removal of the motor then requires some o-rings to be replaced, plus you might need to buy some large Torx bits? I added a few comments on polarity and how I actually carried out the task...
https://www.volvoforums.org.uk/showthread.php?t=206272
For diagnostics I use a cheap ELM327 Bluetooth adaptor (about a fiver on eBay) in conjunction with the free 'Car Scanner' app on my Android tablet. This will read and delete engine/powertrain codes and read a lot of sensor values... I can access the engine oil level sensor value (in mm) and the DPF soot loading (in g).