So, apparently a reservation record can remain accessible in Marriott's reservation system even after check-in and can be "negated" and a cancellation number issued.

IMO the OP should be off the hook and owed an apology and appropriate compensation (and a credit).

And, the folks in customer care inot taking into account that a cancellation had been processed (and the hotel being so obtuse as well), suggests that e for those who were so thick-headed as to dismiss so cavilerly what stood out so obviously as a component in the equation mandatory participation in an "Attitudes-101" refresher course.