As for "I could care less," if you preface that with "it isn't possible that...." it makes perfect sense. I see the expression as an economical way of saying just that, though, arguably, it would be linguistically more efficient, and less likely to have the recipient doing some puzzled mental processing, just to say "I couldn't care less." But here's the rub: both variants are informal and both are widely used, so we just have to suck it up. In formal writing you wouldn't use either unless you were quoting dialogue. We sometimes have to let the lingo take wings and fly, though I'll always argue against pretentious words when there are perfectly good plain alternatives.