Per #6.
You seem to be confusing correlation with causation.
Do you really plan to get up an inner-city posse to combat crime? A posse made up of the criminals themselves? Good luck with that.
I agress 100% that cops often get out of hand. In particular, the ongoing militarization of the police is troubling. But very few people will agree that we would be better off without police. If nothing else, a tremendous number of crimes would go unsolved without forensics, which requires a high degree of professionalism.
Cordoning off a crime scene for collection of forensic evidence, and analysis of the collected evidence, could be conducted by any number of NGO job descriptions. Legal standards for chain-of-custody can exist in a market-based system the same way other technical standards exist in any number of industries.