Not Again!
The joy over the keyboard working again was short-lived -- before long, various keys began ignoring individual keystrokes once more.
I recalled reading somewhere that the original graphite-based conductive layer on the underside of the key plungers could be restored to functionality using conductive silver paint—the kind used, for instance, to repair broken circuit traces. I couldn't find the original source, but I thought it was worth a try.
First, I cleaned the parts with isopropyl alcohol. Then, I applied a thin layer of the conductive silver paint using a brush.
After this procedure, all the keys worked perfectly again. I am curious to see if this solution will last...