Crusader - you dont say how old your Mum & Dad are. This is something that has the potential to make them ill. Id be inclined to go and stay - forget the balaclava etc - your presence might be enough to quell things for a while.
The problem is - the cops wont do anything unless they have proof - realistically they wont do bugger all until a human is harmed.
A neighbour of ours here has had a tree cut down - a witness saw it happening - but it took threats to go to the local cops boss before he would do anything.
You can get CCTVs which are night sensitive I know you have cams up - but changing to night sensitive ones might help (or use both) - and driveway alarms are good too (they can be used in all sorts of places) - buy Mum a diary so she can note down times and dates that things occur.
Get them a big dog as an alarm too