1) As you may have noticed at last night's performance of Mozart's Abduction from the Seraglio at the Met, the role of Konstanze is unsingable, at least in the house the size of the Met.
2) Joseph II's mother, Maria Theresa, had just died in 1780, and partly in celebration of the fact that he now had unbridled control over her hereditary lands (Austria, Hungary, Bohemia, Croatia, Belgium, Lombardy, etc.), Joseph propounded the Edict of Toleration of 1781 (or was it 1782?), in which all loyal subjects might practice their religions in private, including Protestants, the Orthodox, Jews and Muslims. This was brand new -- no European state since the pagan Roman Empire had ever had such a law -- among other things, the Pope (who in those days seldom left Rome) came to Austria to beg Joseph to rescind the law -- he paid no attention. (Joseph also closed all monasteries and convents in his domains unless they were "useful," that is, if they ran hospitals or schools. They're still mad at him in Austria for this.)
Mozart's attitude towards a sublime equality of religious faiths, which was obviously personal since he then joined the Freemasons and wrote The Magic Flute (after Joseph's death), which share this philosophy. Seraglio thus follows not only Mozart's personal beliefs but the current political line.
3) Joseph was trying to hold his hereditary lands together by playing the German cultural nationalism card. He had founded a national theater in Vienna, and the finest actors and playwrights in Germany took part in it; his national opera was an attempt along the same lines, to free Germany from Italian and French cultural superiority. The opera house, however, was a failure - the time was not ripe - and Seraglio, its last premiere, was its only success. When Joseph said "a monstrous lot of notes," he meant it wasn't a simple ballad opera on the lines of The Beggars' Opera (imported from London, and a great hit all over Germany), but had these Italianate flowing lines for Konstanze and Belmonte that could only be sung by very accomplished Italian-trained singers.
He was right.
4) This did not prevent the opera from being Mozart's most popular work in his lifetime, performed all over the Empire. (It didn't reach Italy till 1956. Callas sang the premiere.)
5) Its moralism and its noble Muslim pasha have made Seraglio the most popular opera (with Aida) in non-Christian opera houses. It is performed every year in the Seraglio of the Topkapi palace in Istanbul. But its philosophy is a very 18th-century Rights-of-Man pantheistic multiculturalism. I certainly see it as a splendid counterargument to the "all Muslims are hateful and out to get us" attitude common in this country nowadays (and in Europe too).
6) The quartet at the end of Act II is the first conversational concerted passage in opera that does not interrupt the action and in which all the characters sing characteristically, blending their personalities to advance the drama. It is from this (and the quartet in Idomeneo) that the ensembles of Figaro spring, and with them all of 19th-century opera.