You are wasting your time. Make one phone call and then file a chargeback with your credit card issuer (bank). Time is not on your side here as more than 60 days have passed and, while many issuers will honor a chargeback for as long as a year, there is no legal requirement that they do so.
You should be able to do the chargeback online and you should note that you were promised a refund at your first call back whenever.
No need to let Marriott keep playing games here and potentially run the clock out on you.