I'm thinking now that the oil leak there is coming from the clutch pushrod seal. There's plenty of room around the pushrod, and there shouldn't be.
Anyway I put the chain on, and the master link went on, and when I spun the rear tire there was just smooth motion. The chain made a proper "engaging sprocket" sound as it turned, but was otherwise whisper silent.
The "ass-o-meter" says that the bike is performing better and smoother. I don't know how much is psychological and how much is real.
So, yay me.