You raise an interesting point. Perhaps in the future, anytime someone in street clothes purports to be a police officer our immediate response should be to ask for identification. If they refuse or fail to provide it, we should be on our cell phones reporting someone impersonating an officer.
Of course, a good retention holster is advised anytime we OC. So if we are going to switch between CC and OC, a retention holster should be used.
There are times and places where I'm quite happy to discuss my political and tactical views. There are times and places I'm not. And even when I am, I think it really unwise to get into any kind of heated discussion while I'm armed.
Something along the lines of, "I'm not really interested in discussing this with a stranger right now," to prevent a debate or even "We'll have to agree to disagree," to end one that is going the wrong direction, seems about appropriate as an initial, polite, but direct communication to a busy body. After that, a louder, more firm, "I said, 'I don't wish to talk to you!' Step away from me now or I will call the police!" may well be in order.