Update: I just installed the macOS 10.13 Supplemental update and re-migrated my user account, but with no improvement in the Keychain Access situation.

I forgot to mention at the outset that my login items weren't perpetuated when I upgraded, nor did System Preferences > Users & Groups > Login Items respond to the appropriate check-boxes, and the update didn't fix that either.

The new Great Equalizer is the SEND button.

In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire