It is not a dumb idea at all.I read this whole thread tonight and I have a dumb question. Why wouldn’t you use a m14 to m12 thread reducer in the 2016+ caliper and skip enlarging the holes in the spindle?
It is often used with aluminum calipers that have stripped threads using inserts not for switching thread size but just to fix the thread. Aluminum calipers are notorious for seized bolts that when removed completely destroy the caliper thread.
Search for Helicoil Big-Sert. There is also a lighter version.
Helicoil is a trusted brand used to fix all kind of threads from engine heads to common utilitarian threads. Properly installed with the right tools it will last better than the original. They are available in carbon and stainless steel.
The issue with the adapter is how is the adapter secured inside the caliper. One solution is to use very high strength thread locker for vibrating assemblies and torque it to specs - you actually overtorque it because of the thread locker. Another idea is to weld it on the back. I think both solutions will cut muster.
But the more assemblies you add the more points of failure you add. Drilling is such an easy and cheap job that is the preferred solution here.