Mark, thanks for the switch links. Theoretically any momentary SPST switch will work as long as it's the same dimensions, so that is very useful.

I dismantled my troublesome switch under magnification, and it was a normal dome switch on the inside. It looked like it was manufactured with poor QC, and there was a lot of blackness on the dome and the contact points, as if the black plastic had outgassed or "bled" during the manufacturing. Cleaning it with isopropyl and re-assembling has made it work fine again, but I intend to order more replacement switches just so I have more on-hand so that future repairs are easier.