Short answers, pulled from the story.
Franz Xaver Süssmayr was an Austrian composer and conductor born in 1766 in Schwanenstadt and died in Vienna in 1803 at the age of 37. He is best known for completing Wolfgang Amadeus Mozart's unfinished Requiem after Mozart's death in 1791. He also held conducting posts at the Burgtheater and the Theater am Kärntnertor in Vienna.
Mozart's widow Constanze sought to have the unfinished Requiem completed, but her first two choices, Franz Jakob Freystädtler and Joseph Eybler, did not finish the task. Süssmayr was the third composer asked. His close relationship with Mozart as a student, friend, and traveling companion made him well placed for the work.
Süssmayr composed operas including Moses oder der Auszug aus Ägypten, Der Spiegel von Arkadien, List und Zufall, and Soliman oder die drei Sultaninnen. They were staged at Viennese theatres including the Theater auf der Wieden and the Theater an der Wien, operated by Emanuel Schikaneder. Beethoven borrowed a theme from Soliman oder die drei Sultaninnen for his Variations on 'Tändeln und Scherzen', WoO 76.
Critics gave Süssmayr's work mixed assessments. Max Dies described his style in 1894 as "smooth, pleasing, singable, but generally fashionably superficial," adding that "nowhere does a trace of idiosyncrasy shine out." A biographical entry from 1880 found "neither originality nor poetic depth" in his work while acknowledging its "melody and a charming, even folksy character."
The Süssmayr clarinet concerto, SmWV 501, was most probably written for Anton Stadler, Mozart's clarinetist, and scored for the basset clarinet. It has been recorded by Dieter Klöcker for the Novalis label on standard clarinet and by Thea King for Hyperion using a basset clarinet reconstruction prepared by Michael Freyhan. In 2021, Craig Hill published a completion for period basset clarinet.
Süssmayr received his earliest music lessons from his father, Franz Karl Süssmayr, in Schwanenstadt, upper Austria. He was later educated at Kremsmünster Abbey, where Georg Pasterwitz was among his teachers, and his early compositions were performed at the abbey theatre. After moving to Vienna in 1788, he became a student and friend of Mozart.