These types of incidents are the hardest to determine fault. Make sure to get any possible witnesses to help you out, preferrably people that were not in either car. Usually the person that was the furtherest out is NAF but that is not iron clad.
Depends on the jurisdiction, but typically, two vehicles backing up is a 50/50.
It is your fault.
If they backed into you, it's their fault. Failure to yield.
the car backing out.
You are.
Backing up--it is your fault.
In general, the driver who is backing is considered to be at fault.
The backing vehicle is at fault. We are always supposed to look to ensure the path is clear before placing our vehicle in motion.
The person who drove their car into the parked car.
The person who ran into you from behind of course because they should be stopping, or slowing down when you turn in the parking lot.
The car that is in motion is ALWAYS at fault.
It would be the car who has driven past, then reversed into you.