...can MacOS 10.13.5 be far behind?
I guess I hope not.
My keychain issue was still an issue in a clean install of 10.13.4, and for the first time I tried updating an existing 10.12.6 installation to see if I could make an end run around it, but with no joy.
At that point I remembered that I've still got AppleCare, but the tech to whom I spoke was a new hire and not at all up to snuff. (I'll report back on my next try.)