Short answers, pulled from the story.
Georg Nikolaus von Nissen was born on the 22nd of January 1761 in Haderslev, Denmark, Norway. He completed his schooling in 1781 before entering public service as an authorized agent of the General Post Office in Copenhagen.
Georg Nikolaus von Nissen first met Constanze Mozart in 1797 while serving his diplomatic post in Vienna. Her husband had died six years earlier in 1791 leaving her to face financial hardship alone before they began living together in September 1798.
The couple married on the 26th of June 1809 in St Martin's Cathedral in Pressburg which is now Bratislava. They moved to Copenhagen in 1812 where Nissen took up a post as a censor.
Mozart's elderly older sister Nannerl gave them a collection of about 400 Mozart family letters that provided essential primary source material for their research. Nissen also collected commemorative coins and monuments related to the composer's life alongside written correspondence.
Johann Heinrich Feuerstein a medical doctor and Mozart enthusiast living in Dresden finished the work after Nissen died on the 24th of March 1826. Contemporary scholars like Angermüller and Stafford describe Feuerstein as unstable while Ruth Halliwell judged the final result disastrous in terms of quality.