It’s not grammar and it’s not a correction. The phrase “I could care less” has only one meaning and that meaning is “I don’t care”. It is being used correctly.
Does it? I decode it instantly and understand the meaning just like I know what a "fishbowl" is. There is no "decoding" or even nonsensical input in this case.
You are just being stubborn and trying to adhere to an outdated standard. Upgrade or get replaced.
I agree. I've learned to not care when people say 'expresso' instead of 'espresso', and 'ex cetera' instead of 'et cetera'. I know what they mean, you know what they mean, and correcting everyone only serves to alienate others.