Quick update: my MacBook Air now can type in keystrokes! Yea. But I have zero idea why or what happened to create the change. My iMac however still can do nothing.
At least I know have my wanted macro that includes using command-A followed by command-C followed by several more steps. And the macros sync via iCloud.
... but there must be some way "in the moment" to figure out what's causing this. It seems to be related to "secure input lockout". A couple of folks, a few years ago, wrote KM macros to find what app was the source of the problem. However their instructions via Terminal and/or their KM macro both do nothing for me. Perhaps enough has changed with OS to make those old commands/macros useless. Or I'm doing something wrong. But if I could efficiently find the "cause" of the problem then I could go about fixing things.
This is the thread I saw about the issue from 2015: