Short answers, pulled from the story.
Gregorio Allegri composed the Miserere, a setting of Psalm 51, probably during the 1630s and around 1638, during the reign of Pope Urban VIII. It was written for the exclusive use of the Sistine Chapel during Holy Week Tenebrae services.
According to a letter Leopold Mozart wrote to his wife on the 14th of April 1770, his fourteen-year-old son Wolfgang heard the Miserere during a Wednesday service in Rome and wrote it down from memory the same day. Doubt has been cast on the story because Mozart had previously visited London, where the piece was regularly performed, and because Leopold's letter contains confusing and seemingly contradictory statements.
Pope Clement XIV awarded Mozart the Chivalric Order of the Golden Spur on the 4th of July 1770, less than three months after Mozart's famous visit to hear the Miserere.
The "top C" appears in the second half of the four-voice falsobordone and is the defining feature of the version most commonly performed today. It traces to the edition published by William Smyth Rockstro in the first edition of the Grove Dictionary of Music and Musicians in 1880, later combined with the first verse of Charles Burney's 1771 edition by Robert Haas in 1932.
The Choir of King's College, Cambridge recorded the Miserere in March 1963, conducted by David Willcocks, with the then-treble Roy Goodman among the singers. The recording was originally part of a gramophone LP called Evensong for Ash Wednesday.
Allegri's Miserere is written for three choirs, a five-voice choir, a four-voice choir, and a third choir singing plainsong responses, all alternating and joining for a nine-voice finale in verse 20. A full performance runs between twelve and fourteen minutes.