Declining to explicitly sign up for and pay a supplement for insurance coverage may be required for the Amex insurance coverage to be applicable, but IIRC it's not required that the car rental itself only be made to exclude all potential insurance coverage that may come as included with a car rental rate plan without a supplemental line item charge. But it's something you should ask UK Amex about.

I redeemed Hertz points to extend the validity of the account's points before, and then cancelled the points reservation later. Maybe something to consider if a useful objective is to buy time to keep the Hertz points/account alive.