Fourth, if private security violates your rights you can sue. When government violates your rights they typically get the case dismissed due to 'qualified immunity' which USSC made up in 1970s.
I am curious if there is case law to support your theory. It would be hilarious to read about a city or county attorney trying to file a motion to dismiss, when the city or county would not be a party to the case.
No need to look to the future: this has been going on for a long time and is pretty common. Even though case law has built up, to my understanding it still gets pretty messy.