Part of the problem was that the output of my “trace bitmap” was tracing both sides of the solid line, and therefore producing two lines that were very close together.
I solved the problem by upgrading Inkscape (from 0.92 to 1.0), and then using the new
“centerline tracing (autotrace)” option (see final screenshot). Then I still had to reduce my line weight to 0.1pt.
now it works as expected, with a single pass, single trace cutting.