Could you please post those Ts and Cs? Here is what I find when I look at the Chase Sapphire auto insurance:
Auto Rental Coverage
Your Auto Rental Collision Damage Waiver has been enhanced from secondary coverage to primary. So if the unexpected occurs, you can now rely on an enhanced level of coverage. Decline the rental company’s collision insurance and charge the entire rental cost to your card. Coverage is primary and provides reimbursement up to the actual cash value of the vehicle for theft and collision damage for most rental cars in the U.S. and abroad.*
American Express car rental insurance policies can be found
here for their family of cards.
In my experience with car rentals, you have to use the same card to pay as you use to pick up the card. You have to abide by the credit card rules, so declining the insurance that the car company offers. Autoslash does not use a credit card to hold the car rental. Rather, you give a credit card when you pick up the car.