Short answers, pulled from the story.
Gregorio Allegri held his post in the choir of the Sistine Chapel at Rome from the 6th of December 1629 until his death on the 18th of February 1652. He obtained this appointment after Pope Urban VIII recognized his sacred music works.
Gregorio Allegri published two volumes of concerti for five voices during 1618 and 1619, followed by two additional volumes of motets for six voices in 1621. His known output also includes an edition of a four-part sinfonia, five masses, and two settings of the Lamentations of Jeremiah.
Wolfgang Amadeus Mozart transcribed Gregorio Allegri's work faithfully from memory during a visit to Rome in 1770, creating the first known unauthorized copy. Dr Charles Burney subsequently procured and published Mozart's version in England in 1771.
A strange error introduced by a copyist in the 1880s caused a curious trucker gear change from G minor to C minor because the second half of the verse is the same as the first half but transposed up a fourth. The original composition never contained a Top C note, yet modern editions often reflect these historical mistakes rather than Gregorio Allegri's original intent.