in my experience renting cars, a hold is placed on a CC when the contract is opened, and then when it is closed, the charge may be processed either on the card on which the hold was placed, or on another card, at the discretion of the renter.
As far as I can see, one would be able to use an AmEx card initially on the contract - to get the premium primary insurance for a fee of $25, and then when closing the contract - assuming the bez"H the trip was uneventful and the insurance is not actually needed, one could actually charge the rental price to a different card, if for example one had a min. spend to reach on a non-AmEx card, and the hold that had been placed on the AmEx would just drop off.
Result:
No need to take out expensive insurance from the rental company - covered by amex, and if all goes smoothly actual rental cost charged to a non AmEx card, and if G-d forbid there are some hiccups and the insurance is needed, it is there as the car has been rented using a hold on an Amex - exactly the way it would be if one were not planning to change cards later.
It just seems too simple.
Am I missing something?