I completely removed the rear spoiler, applied VC3 to all bolts, applied 3M tape to contact areas of the black little "winglets" on the sides, and bolted everything back down.
The vibration noise continues minimally abated.
This is a design flaw by Mazda, caused when the black "winglets" flutter against the rear window under certain resonances. The only fix I can think of is to use black floor protector stick-ons applied to the window under the tips of the winglets. This is removable and "low impact" and will solve the issue 100%. Of note, when I removed the spoiler, the bolts holding the winglets on were NOT loose. Nor is there any 3M tape anywhere in the assembly, contrary to what you may have seen on online diagrams.