The rule of thumb is one call or email, followed by a chargeback if you do not see the refund in 7 business days.
You are wasting your own time making multiple calls, sending "Mr. Marriott" emails and the like.
Thirty seconds of your time online and you will have a temporary credit in a day and that will become final soon enough.
Not sure why people worry so much about chargebacks. This is exactly the situation where they are effective and efficient. Also, do not forget that while most card issuers permit longer periods, you only have statutory chargeback protection for 60 days and there is no reason to risk that.