Short answers, pulled from the story.
Friedrich Schlichtegroll published Mozart's obituary in 1793, making it one of the earliest biographical accounts. It appeared in a volume of obituaries called Nekrolog and drew primarily on information from Mozart's sister Nannerl and family friend Johann Andreas Schachtner.
Since Maynard Solomon's research in 1991, scholars have concluded that Rochlitz's stories are contaminated by his own fictional additions. Rochlitz edited the Allgemeine musikalische Zeitung, a journal tied to a Breitkopf and Hartel edition of Mozart's works, giving him commercial motivation to produce entertaining rather than accurate accounts.
Constanze was a primary source for several early biographers and was married a second time to Georg Nikolaus Nissen, who wrote one of the early biographies. Modern scholars including Cliff Eisen have expressed skepticism about her accounts, noting she had strong incentives to present Mozart's final years tragically because she was seeking an imperial pension and income from memorial concerts.
Tyson mastered watermark analysis to date Mozart's manuscripts, while Plath analyzed handwriting. Their independent methods produced converging dates for Mozart's compositions on nearly every occasion, which scholars took as evidence that each method was more precise than its proponents had claimed.
Michael Lorenz's archival research revealed that Mozart was living in spacious, expensive suburban quarters at a period when conventional scholarship had assumed he moved to the suburbs because of poverty. Lorenz also established the correct identity of the person for whom the Ninth Piano Concerto was written.
Nissen was Constanze Mozart's second husband. After retiring from the Danish civil service and moving with Constanze to Salzburg, he received a large collection of Mozart family letters from Nannerl. He died in 1826 before finishing the biography, and others completed it from his notes in 1828.