Short answers, pulled from the story.
Franz Xaver Niemetschek (the 24th of July 1766 - the 19th of March 1849) was a Czech philosopher, teacher and music critic. He is best known for writing the first full-length biography of Wolfgang Amadeus Mozart, published in 1798.
Niemetschek's first Mozart biography was published in 1798 under the title Leben des k.k. Kapellmeisters Wolfgang Gottlieb Mozart. A revised version appeared in 1808 with the title Lebensbeschreibung des k.k. Kapellmeisters Wolfgang Amadeus Mozart.
Niemetschek claimed a long personal association with Mozart, but scholars have questioned this. The biography contains no direct quotations or records of personal conversations, and research by an Austrian scholar has recently cast serious doubt on whether he made Mozart's personal acquaintance at all.
Niemetschek welcomed Mozart's two surviving sons, Karl and Wolfgang Jr., into his home in the Lesser Quarter of Prague and became a foster father figure to them.
Niemetschek is buried in St. Marx Cemetery in Vienna, where he died at the age of 82.
Niemetschek taught poetry and Latin at the Gymnasiums in Plzeň, and from 1802 he was professor at Prague University, where he lectured on logic, ethics and pedagogy. The composer Jan Václav Voříšek was one of his students.