Nitpick, but I think you mean “can’t be overstated” - no matter how important you state the thing to be, it’s not overstated, because the thing is just that important. Hence, can’t be overstated.
Not to argue about idioms, what you say is right.
How did "couldn't" become "could"?
It inverts the whole meaning of the statement.
Sarcasm and Laziness are at least 2 reasons people might modify it. The second carries it through time.
Saying that you ‘couldn’t care less about X’ doesn’t seem logical at all because you of course could, ‘could care less’ is of course not particularly informative unless you’re expressing intention or a desire to care less about something.
I think the whole point is that it means you don't care at all, and therefore it's not possible to care less.
The moment you say “couldn’t care less” you’re already technically incorrect because you obviously could..