I didn't get the Leopold one, I'm afraid, or any of the Irish subplot. I did vaguely remember reading about an opera that is the source of the "Jewels of the Madonna" idea, but I couldn't tell you which one without looking it up in my Kobbe.

(I discovered Kobbe's Complete Opera Book when I was at school. There was a copy in the reference library. I have my own copy, now. :))