Short answers, pulled from the story.
Georg Nikolaus von Nissen was a Danish diplomat and music historian born on the 22nd of January 1761 in Haderslev, Denmark-Norway. He served in the Danish foreign service from 1792 and was posted to Vienna from 1793. He is best known for marrying Constanze Mozart, the widow of composer Wolfgang Amadeus Mozart, and for authoring one of the first biographies of Mozart. He died on the 24th of March 1826 in Salzburg.
Nissen met Constanze Mozart in 1797 in Vienna, initially as her tenant. He became deeply involved in managing Mozart's literary and musical estate, negotiating with publishers on Constanze's behalf. After retiring to Salzburg in 1820, he began serious work on the biography in 1823. He drew on roughly 400 Mozart family letters given to him by Mozart's sister Nannerl, on interviews with people who had known the composer personally, and on Constanze's direct testimony.
Nissen died before completing the biography and only the preface can be fully attributed to him. Completion fell to Johann Heinrich Feuerstein, a physician from Pirna, whom scholars described as unstable. The finished book drew from earlier accounts of dubious reliability, contained contradictions and errors, and included letters that had been selected and censored. Nissen himself also edited Mozart's letters in ways later scholars sought to reverse. Scholar Maynard Solomon also argued that Constanze validated false reports in the biography about Mozart being exploited and impoverished.
Nissen explained in his foreword that he believed a biographer must protect the reputation of the subject by selecting carefully from personal correspondence. He wrote that showing a hero in the private moments of familiarity could harm his fame and the impression of his works. Later scholars called this approach bowdlerization but generally accepted his intent was protective, not deceptive.
The biography was published posthumously in 1829 under the title Biographie W. A. Mozart's. Nach Originalbriefen, Sammlungen alles uber ihn Geschriebenen, mit vielen neuen Beylagen, Steindrucken, Musikblattern und einem Facsimile. Some sources note the title page bears the date 1828.
Nissen died aged 62 in Salzburg, where he was buried. His tombstone identifies him as 'The husband of Mozart's widow.' The inscription reflects how closely his identity became tied to Constanze Mozart and to Mozart's legacy. The tombstone can still be visited in Salzburg today.