Tried to edit my comment but you'd already replied :D

selenak suggested I link you -- here are my reactions to watching Un Ballo in Maschera (A Masked Ball), which also double as a synopsis, more or less. In the comments zdenka also links to a synopsis from her DW, which is a better synopsis and probably funnier as well :)

Basically, the plot is actually the history you have delineated in this thread, except that Verdi added a love triangle with a best friend (who is obviously my favorite! :D <3 ), who ends up joining the conspirators.

Also, Gustav in the opera is (apparently at least somewhat intentionally) written as totally bi, although I didn't clue into that when I first watched it (the direction didn't really make a thing of it).