Depends how long it has been there.
Have it towed?
The driver that hit the parked vehicle would be at fault.
In the UK it can, if the driveway is private property.
if the vehicle is parked on private property and if there is a sign saying that vehicles will be towed away you can move them.
Yes you can, as long as its not stolen.
If the vehicle is parked on a public street, you need a tag. If it is on private property you don't.
The person who caused the damage.
The person who hit a parked vehicle is at fault.
As a general rule, a parked or stopped vehicle is almost never at fault for an accident. The onus is on the driver of the moving vehicle to make sure the way is clear.
The individual who had custody over the unattended, parked vehicle would be at fault. Clearly, the vehicle was not adequately parked (or not up to safety standards) if it rolled down a hill and damaged persons/property.
I am renting a trailer and a limb falls on my vehicle while parked on the property who is responsible for the damages
If it's parked on public property, or you're caught operating that vehicle, yes.