I think #6 gets a pass. I never use it in writing, and the grammatical analysis is unassailable. But "could care less' is not really a phrase, but a collection of words. It is a meaningless unthinking utterance used in practice like something said if you hit your thumb with a hammer or have an orgasm with a hooker. Thus it qualifies under the inane cliche exception to the speech is meaningful rule. Other common examples are bad=good, hilarious=interesting, democracy=American political system.