Very interesting discussion - I am intrigued by Zhdanov's explanation for Trovatore's bizarre backplot. It's plausible, and like others here I'm also curious whether or not the source play made this explanation clear, as well as whether or not Verdi's contemporary audiences understood it this way.
At the risk of stating the obvious, we can't leave out the near-fatal flaw in the three great Mozart/Da Ponte operas: the idea that people can put on a disguise and become unrecognizable (even at seduction-level close-up) to the people who know them best. It just seems like this would be really hard to pull off in the real world. Maybe candlelight made it easier.