In my experience Amex either lies intentionally or is misinformed. I have downgraded several Amex cards after receiving the companion cert and they all threaten me that the companion cert will be revoked but this never happens. I continue to use it and it is perfectly valid. So once you have the companion cert feel free to use it and complete travel prior to the deadline. Any 2 people can use it but you must use an Amex card to pay for the flights.