Because the top layer of the front cross-member admitted a gap, I broke the isolating cut into two sessions. Yesterday evening, I cut through the front half, in the hopes that the short was there. It wasn’t so, after the epoxy holding a new frame sandwich together had cured, I severed the back half.
I almost feared touching the two sides of the frame with my meter’s probes. If this was not it, there was nothing else to do but redesign the frame and start over. What a relief it was to see the display stubbornly stuck on “1,” indicating an open circuit. I’ve now made a second sandwich of styrene and the newly isolated frame halves, and clamped it shut with a healthy amount of epoxy.
The plastic rods protruding from the sandwiches will get cemented to their adjacent styrene sheets after the epoxy cures. Two types of glue is better than one.
Pembroke:87 
I can almost feel the sense of relief that comes with that success. Congratulations!
Thanks Rob. There’s still more recovery to go, but at least I will be going forward now!
So was it the etching shorting or the epoxy?
We’ll probably never know!