Short answers, pulled from the story.
Franz Xaver Süssmayr entered the world in 1766 within the quiet town of Schwanenstadt. His father, Franz Karl Süssmayr, provided his first music lessons before he moved to Kremsmünster Abbey for formal training under Georg von Pasterwitz.
Mozart died later in 1791, leaving his widow Constanze anxious about finishing the unfinished Requiem Mass. Constanze eventually entrusted the completion task to Süssmayr after other composers failed to satisfy the family's needs.
Süssmayr served as director at the Theater am Kärntnertor from 1794 until his death in 1803. He also became substitute conductor at the Burgtheater starting in 1792 while staging operas alongside Emanuel Schikaneder's theaters like the Theater auf der Wieden.
Max Dies described Süßmayer's writing style as smooth, pleasing, and singable yet generally fashionably superficial in 1894. Unbiased critics found neither originality nor poetic depth even though friends called him a second Mozart.
Süssmayr composed Soliman oder die drei Sultaninnen which Beethoven later used a theme from for his Variations on Tändeln und Scherzen. Another ballet premiered at La Scala on the 25th of April 1812 and inspired Niccolò Paganini to create Le Streghe.