I apologize for posting that… I was jumping to conclusions. I’m new to Mac so I am not familiar with uninstall procedures. (On Windows generally one runs the “uninstaller” and the program is gone.) What I learned is that Karabiner-Elements installs some daemons and user-agents, and I needed to delete those plists before it stopped messing things up. It’s fine now.
What it did was mess up OS X’s ability to recognize external keyboards. In fact, when I tried to identify a particular keyboard, OS X told me that it was the internal keyboard. This seemed so “buggy” to me that I figured something had been deeply corrupted, leading me to post the above.
I didn’t realize this kind of behavior could result from the K-E daemon still running until later. I didn’t even know K-E had installed a daemon.