All's well that ends well.
Our mechanic took off the starter and starter adapter. He confirmed that there was nothing wrong with the starter adapter, which is a big relief, although he did give his personal opinion about the design of the thing!
The starter motor had failed - possibly an internal short, since when the starter relay was energized, the voltage across the starter, dropped to 8 volts.
So thanks again for all the input. I'll be much more aware when starting TCM engines from now on.