Stubbornness.

It pays off.

As I mentioned, I have a hard time accepting that I can't fix a problem. So I kept searching. A few promising help articles indicated improper grounding as the source of the trackpad behavior. After using someone else's (newer) charger, the problem went away.

Poof. Gone.

Crazy. What a day. My hopes are still hedged by the fear of a future relapse, but so far the outlook is promising.