Reading the other posts on "walks", being notified in advance precludes this being considered a walk and invoking the Platinum Guarantees. Also, depending upon state laws, hotels may have the ability to void a reservation with prior notice and be exempt from liability.
However, I would push the issue up to Marriott corporate and see what they are willing to do on your behalf. I have a friend that was booked in Cancun. The property had delays in recovering from hurricane damage. Hotel tried walking away from the reservation. Corporate got involved and he was hosted in a neighboring hotel (not Marriott) for the week.