Short answers, pulled from the story.
Joseph Lange was an Austrian actor and amateur painter who lived from 1751 to 1831. He became Mozart's brother-in-law in 1782 when Mozart married Constanze Weber, the younger sister of Lange's wife Aloysia Weber.
Constanze Mozart, interviewed in old age by Vincent and Mary Novello, described Lange's portrait as "by far the best likeness" of Wolfgang Amadeus Mozart. Lange painted it in 1782-83.
The portrait began as a miniature approximately 19 by 15 centimetres showing only Mozart's face. Musicologist Michael Lorenz was the first to establish this in March 2009, when he recognised that the small painting had been affixed to a larger, unfinished canvas.
In 1783, Lange performed in a Mozart Carnival piece known as a Masquerade (K. 446), playing the Commedia dell'arte character Pierrot while Mozart himself played Harlequin. In 1786, Lange appeared in Mozart's opera Der Schauspieldirektor, taking the spoken role of Herz.
Aloysia Weber was a successful soprano whom Lange married in Vienna on the 31st of October 1780. She was the sister of Constanze Mozart, making Lange and Mozart brothers-in-law; Aloysia also performed alongside Lange in Mozart's 1786 opera Der Schauspieldirektor, singing the soprano role of Madame Herz.
Lange published his autobiography in 1808. He continued painting into old age and died in Vienna on the 17th of September 1831.