Update: I installed a brand new battery. I turned on the headlights and dome light and honked the horn - everything worked just fine. I then pushed the floor starter and could hear a faint "crackle" or "sizzle" coming from the battery. And the lights/horn wouldn't work after that. I wanted to see if I could replicate this issue... so I disconnected the battery, reconnected it a few minutes later. Again, the lights and horn all worked, but then I pushed the floor starter and hear another faint "crackle" sound and nothing electrical worked after that. No starter, and no lights/horn to follow.
Could this be the starter switch causing a short circuit when depressed? I inspected the starter and the "button" that is depressed by the floor starter linkage, and they both looked "normal", ie nothing caught my eye (take that for what it's worth).
Finally, I put a multimeter on the battery's hot *pole* and it read great. But when I put the multimeter on the hot cable (not pole), I got nothing. There seems to be something in the starter that shorts out and prevents current from flowing whenever the starter switch is activated.
Does this sound like a bad starter switch? If so, is there any way to address this without removing the starter itself? (I'm guessing not... but figured I'd cross my fingers and ask anyway).