Even though I am not in the same bind, I feel your pain. My suggestion is document all of your attempts to reach out to them and to use the certificate before it expired. Then if you are unsuccessful at reaching Marriott to get it extended or attached to a reservation in time, give it a little bit of time and try reaching out to them after the integration is completed this coming weekend. Hopefully they will be reasonable and retroactively extend your certificate for you since the problem was created by the pre-integration issues and their glacial response time. No guarantees of course, but if you ask politely and firmly after the integration, I think there is a good chance they will grant an exception since the problem was mainly on their end and they should be aware of that. Hope you get through to them beforehand. Regardless good luck and hoping for a good outcome for you!

--Jon