The culprit appears to be the headlight hi-lo switch in the turnsignal stalk assembly on the steering column. 

Since all accessories work with the ignition on or running, I ruled that out.

I carefully tested the dash light switch for continuity and resistance, particularly terminals 56 and X.  All good.

When I jumped the white/yellow-black/white-black wires at the hi-lo switch, the headlights worked, more or less, both hi & lo. I have the SA inner highbeams disconnected to reduce load. 

Question: does the black male connector on the left upper side of the steering column - marked 56 56b 56a 90 - detach from the turn signal assembly? Could its upper contacts be part of the issue? I can't find that as a separate part, yet it does't appear in pictures of the turn signal assembly.