So, I'm checking back in to update you guys on what ended up happening. After several times of pulling the starter and successfully bench testing it, only to have it not start when I put it back on...as well as many other tests that involved re-routing wires and jumping terminals and cleaning grounds and replacing the ignition switch...and blah blah blah...
My dad (who is usually never stumped) said "have someone turn the key and reach under there and tap it with a screwdriver," which sounded awful familiar to some advice I'd received on here
...and I'll be damned if it didn't fire right up. Pulled it off and bought a new one that night and she's been starting strong ever since.
If someone...ANYONE...can learn anything from this, I pray it saves you the frustration!