This all encouraged me to try again with the elevator-only map. Made various changes, but burying the aas_obstacle fully within the aas_solid broke the log jam.
The good news: the AI guard can fetch the elevator from both levels.
The bad news: using the elevator leaves him KO'd or dead!
Specifically, he fetches the elevator from the top floor, rides it down, makes his rounds on the ground floor, fetches it again and rides it up. All good. But when it reaches the top, the platform immediately reverses and heads down again, inflicting pain in the process! When it gets to the bottom, the AI pushes the top-floor button, and the cycle repeats... more pain. And after yet one more cycle, he's flat and twitching.
Morale: be careful what you wish for?
Silver lining: possible new way to disable guards?
So the mystery continues