Also, I thought about "kindly informing" the owner about the police presence but they do not care what goes on here. Also, in their eyes I am the bad guy. :grumpy:
I understand that the owner doesn't care and thinks you are the bad guy. That's why you need to make them care, and at the same time start looking like the good guy. If you ring them often enough - especially in the middle of the night - they will start caring. You need to make this their problem, because they are the ones who can kick out the neighbors.
If you ring to 'kindly inform' them, you look like a taddle teller and somebody who is sticking their nose in other people's business. However, if six police cars show up in your neighbor's drive, you have a right to be concerned about your safety, and find out what's going on. If you ring the landlord inquiring and acting concerned, they might still be pissed - and that might not be a bad thing - but they can't really hold it against you. And this doesn't get old - every time the cops show up, you have a valid reason to be concerned. Also every time the noise level becomes unbearable, you have a reason to be concerned.
If I had to call them I would start something like this:
"Hello Mr Blahblah, I am so sorry to have to call you again, but there are six police cars in the drive again, and I am really scared. I just don't know what illegal activities your tenants are involved in. Do you have any idea what's going on? Is there any need for me to be concerned?"
Also, if you have never mentioned the garden hose cutting incident to the police, you might want to have a chat with them about it at some stage. It is not that easy to cut a garden hose with a set of nail clippers or a butter knife, so I figure the neighbor must have come out with a fairly substantial knife or a big pair of scissors. If somebody came rushing in my direction with any of those instruments, I would feel threatened. If so, tell the police about that. If the neighbors have domestic disputes on a regular basis now, you could help the police establish a history of violent behavior, which could mean one of the neighbors will be removed - at least for a while.
In addition, a good long garden hose can easily cost $50, and property related incidents are generally considered more serious under US law than, for example, a noise infringement.
Last but not least, always be friendly and respectful toward the police. If you call them, thank them profusely for taking the trouble to come out, even if it takes them an hour to show up. Emphasize with them for having to deal with this kind of crap, apologize for having to involve them, and tell them about your situation in a factual manner. From what I hear, you are going through hell there, and they should feel sorry for you. If they do, they will go the extra mile to help you.
Police officers are used to dealing with totally unreasonable people. Unless you convince them otherwise they have a tendency to suspect you might be unreasonable. Many folks would expect them to come out and solve the problem once and for all. Since the police can never do that, those folks get angry at the police. You don't want to fall into that group. Indicate that you understand that this will be a long process - it has been already! As others have suggested, ask the police to document stuff for you, but indicate that you need this documented so you can eventually get the neighbors evicted, not to built a case against the police for not helping you. Expressing your gratitude for the police officers' efforts will go a long way in this.
Also, if you can afford it, get advice from a good lawyer. Good legal advice is priceless, but if you get advice from a crappy lawyer, it's worse than getting no advice at all!
Best of luck!