It's been a full week now. Cotton is showing signs of interest in what's happening outside his glass enclosure (or else he's plotting his escape).
Still the occasional outburst of angry swipes at my hand (always safely on the other side of the glass). Lots of growls & hisses, but that seems to be his regular tone of voice.
He now approaches the glass when I enter the room to visit. Still not really reaching out like a 'normal' cat, but he is significantly better than when he got here.
Oh, and whatever pitch or tar or engine gunk he had on his ear has finally come off. Guess it wasn't a wound after all. That's good, at least.