I'm so sorry you're going through this. 
I'm a police officer and if this landed on my desk I'd be taking it very seriously. I can see that there would be a lot of evidential difficulties running this as a case, but if it was mine I'd still be initiating a load of safeguarding actions even if I thought I wouldn't be able to get a prosecution or even arrest.
Are the police going to fingerprint the card?
What is the wording on your non-mol? Is he attending your parent's place because it's not technically a breach? You can apply to have it varied. Even if it's not a breach, do you have any evidence it's him responsible for the card/alcohol through letter box? You mention your neighbours. If they confirmed that to me I'd be arresting him on a new allegation of stalking with fear of violence and seizing all his electronic devices (they can be checked for location as well as sending messages etc).
Police can help with placing extra security features on your house and CCTV if you're deemed high risk. They should be doing a specialised stalking risk assessment (not just the standard one for domestic abuse which doesn't always reflect the risk of stalking adequately). Ask them if it's not offered. Sadly, it would be doubtful that they would do this for your parent's house, however.
At your parent's house, consider a fireproof letter box or if you can't afford it, get a lockable postbox outside and seal the original letter box.
You can buy wireless 'spy' cameras for as little as £15 online if you want to dot a couple around on windows.
I don't want to alarm you but have you checked your car for tracking devices? Also, check your phone and change all your log-in details (don't newly block him though, unless he's already blocked by you, as this can spark an escalation in behaviour).
Download the Hollieguard app on your phone (it's on Google play or apple store).
Check out Paladin's website (advice on how to deal with stalking and harassment).
Don't let yourself be fobbed off and ask to speak to a supervisor if you feel your PC isn't responding quickly enough. Asking to speak to a supervisor isn't making a complaint so don't be worried about getting anyone into trouble unnecessarily.
Hope things get better. 