I take "I could care less" to mean "I could care even less about this than I already do, that's how little it matters to me", but that could just be me trying to rationalize a mangled expression.
Well, yes, but it's always possible to care less about something, unless you are already at the state of absolute-uncaring. Therefore, "I could care less" does nothing to express the desired sentiment.
"I couldn't care less" is a much clearer reference to being at this state of absolute-uncaring.
What you said exactly makes no sense. If you said "I could NOT care less" it would mean, you care so little that it is not possible to care less than you do currently.
Regardless, I think this is just a result of people getting lazy with their language... it is not meant to be said in this say, and is properly stated, "I couldn't care less". In the US (can't speak for elsewhere) people just repeat what they think others have said, to convey what they think they understood. This is the source of a lot of language rot.
I think the intended meaning by those who say it is: I'm currently caring too much because I'm talking about it even a little bit. I could easily care even less than this.
