You just confirmed you didn't get the point, saying :
"incredibly vast majority of users don’t shoot flat copy work at close distances".
That's not the point, it's just more visible and reproductible there.
"critical elements in the extreme corners"
That's still not the place where the problem starts.
"The center of the 16 PM is at least as sharp as previous models"
The center, yes. But just when you get out of the center, the image starts to deteriorate.
And if you knew the rule of thirds (that you may not know according to what you say, just search it to be informed), you should know that the near center IS the place where critical elements can be, and where the problem starts to be really noticeable.
You seems to deliberately ignore that talking always of the center and the far corners to evacuate the problem.
"If your lens is defective"
The problem is not my lens, it is about 90% of the lenses, including those in Apple Stores.
It's not because yours is in the 10% that the problem does not exists.