I ended up with a solution to my audio hum problem.
The iMic is out of the loop which means I can’t use Final Vinyl: it won’t run if it doesn’t detect an iMic in the loop. Using it made too much noise, worse than before. So we’re working with the old dependable: Audacity.
The turntable is running into an old receiver and the output from that to the line-in on the old iMac. Works fine, as best I can tell. The input volume in Audacity has to be way down, but I have it playing through the speakers and it sounds just fine.
Sluice down the vinyl with spritz or two from a spray bottle of clear water, run round it with an old DiscWasher brush, and drop the needle. From what I can tell, leaving a thin film of water eliminates some crackling and popping, either by lubricating the needle’s passage or by eliminating static and dust by negating the natural magnetism of a vinyl record.
<later the same evening> It looks like Audacity has what it takes to get this job done. It’s easy to walk through the resulting waveform and insert some labels (for track splitting), and then export the multiple tracks as a single task.
Audacity itself is extensible with a Lisp-derived plugin language called Nyquist, and I found a plugin that claims to be able to insert labels each time it finds silence. Trouble is, we don’t agree on a definition of silence. So I’ll do it by hand for the time being.