YAY!
Marriott and SPG were completely unhelpful over the phone so I decided to risk it and just cancel/rebook myself and hope the points and booking went well. I had the Prince de Galles in Paris booked for 5 nights @ 120k starpoints. I cancelled online just now and the 360k marriott points refunded immediately; I was then able to rebook at 240k marriott points for the same dates.
I hesitate to really suggest anyone do the same (cancel/attempt to rebook) because the IT is still wonky, but if you're at your wits end, tired of stressing about it, and don't feel like waiting around for the hotel to sell out maybe it's worth a shot to just try it yourself online. Good luck everyone
EDIT: to be clear I had made the original reservation on Aug 8