I honestly just use jumper wires and a voltmeter.
Something that plotted multiple switch states as a function of time (like a multi channel o scope) would be useful.. or even something that visualized the state of 5 or so things as lights (simulating the switch test you get in a modern pin) could be useful.
I don’t know if old tone generators would actually work well in an em to trace wires... but that would be useful due to the horrid fading common on most cloth wiring.
Something that easily clipped onto tabs to replace a switch could be useful for troubleshooting... as are your ideas about visualizing or using sound.