Unless something has changed, I believe the AMEX car-rental insurance is secondary (in excess to your own coverage). If your own insurance policy provides any benefits for US market, they will pay first with AMEX picking up what your own insurance does not pay.

I've never heard of any credit card offering liability coverage. If you find one, I'd love to know about it.

Isn't AMEX the only insurer with the clause that you must pay the car rental in full (coupons void the coverage)? I know charging even taxes and fees on airline reward tickets to Chase CSP/UA MP Explorer/UA MP Club card triggers trip interruption/cancellation insurance. I would assume the same applies to car rentals charged to said Chase cards.