1. If it were in Europe, that may complicate it a little. However, I'd have thought it'd be much more straight forward if in the UK and you caught the incident. The only exception I can think of is if the third party were not insured which is a possibility.
2. Most cameras that have parking recording (not all do) will have a G sensor that detects knocks. Sensitivity can be configured on mine. Unfortunately, closing the door is enough to trigger it when leaving the vehicle. When I return to the vehicle and power up the car, it announces the number of incidents detected in parking mode. If it's more than one incident, I can go check the car. One can collect the footage via mobile phone, or taking out the memory card and inserting into the PC.
There are some 360 degree dashcams on the market but I've no experience with them. My front and rear cam is better than nothing. A good bump to the side door would usually render an incident, a recording of them parking up, and hopefully enough to confront them or an insurer with.