Semantic drift: are the words "can", "could", etc becoming contranyms?

Can help (but) is a Negative Polarity Item,
and so was could care less until it caught negation by association, like (could) give a damn.

An NPI is a word (e.g, ever), phrase (in weeks/months/years), or grammatical oddity (would care to) that only occurs grammatically inside the scope of a negative word. There are a lot of negative triggers.

• *She said she would care to have a gin fizz.
• She said she would not care to have a gin fizz.
• *She said she had tasted a gin fizz in years.
• She said she hadn't tasted a gin fizz in years.
• *She has ever been to Chicago.
• She has not ever been to Chicago.

No, there's nothing sinister going on with can and could that isn't going on with all the other modals, too. This is what you should expect with a modal and a negative in the same sentence. Negation and Modality are plenty complicated individually, plus modals and negatives always interact, in unpredictable ways.