Short answers, pulled from the story.
Friedrich Schlichtegroll (the 8th of December 1765 - the 4th of December 1822) was a German teacher and scholar who wrote the first biography of Wolfgang Amadeus Mozart in 1793. His roughly 6,000-word account appeared in a volume of twelve obituaries titled Nekrolog auf das Jahr 1791, published two years after Mozart's death.
No. Schlichtegroll never met Mozart. He gathered his information through a chain of intermediaries: a Salzburg friend named Albert von Mölk contacted Mozart's sister Nannerl, who also forwarded a letter of memories from family friend Johann Andreas Schachtner.
Schlichtegroll's sources, Nannerl and Johann Andreas Schachtner, had little close contact with Mozart after 1781, the year he left Salzburg for Vienna. As a result, the biography is weighted toward the first twenty-five years of Mozart's life, when he still lived in Salzburg.
According to the Grove Dictionary of Music, Constanze bought up and destroyed the entire edition of the Nekrolog, apparently disliking its portrayal of her. The biography's harsh treatment of Constanze reflected the antipathy Mozart's sister Nannerl felt toward her.
Scholars Cliff Eisen and Simon P. Keefe identify Schlichtegroll as the originator of a tradition depicting Mozart as sublime in music but inadequate in worldly affairs. Researcher Bruce Cooper Clarke described this as an "eternal-child" myth that has intruded on every subsequent effort of serious Mozart biography.
Two key sources from Schlichtegroll's research survive today: Nannerl Mozart's written replies to his queries, and a letter from Johann Andreas Schachtner filled with childhood anecdotes and personal memories. Both documents can still be read.