After a day or so of a break, I sat down again with some Internet search results, my headphones, the stereo jack, and its schematics. I just needed to clear my head. I am pretty sure I understand how it works now, and that I can solder only the 3 pins I need for the circuit schematic.
But I figured I may as well buy a handful of mono jacks in the same order as the switches I need. I’m sure the extra stereo jack will come in handy in another project!
And of course, with a cooler head (and different tool), I managed to get the snipped mono jack out of the case. I’m trying not to be miffed at myself for being so rash as to cut it off without thinking through the stereo jack problem. It’s just more experience, I suppose.
So! I’m waiting for that order plus a new solder fume extractor before I solder leads to the trimpot and mono jack.