Short answers, pulled from the story.
Johann Andreas Stein (the 16th of May 1728 - the 29th of February 1792) was a German maker of keyboard instruments based in Augsburg, regarded as a central figure in the history of the piano. He is primarily responsible for designing the German hammer action, known as the Prellzungenmechanik, which gave players remarkable control over the hammers and was praised as a breakthrough in the piano's history.
The Prellzungenmechanik, or German action, was perfected by Stein around 1780. Each hammer was mounted on top of its key as an asymmetrical lever; when the key was pressed, a small beak on the hammer engaged a sprung escapement hopper attached to the keyframe, causing the hammer head to fly upward and strike the string before falling back, whether the key was held or released.
Mozart visited Stein in Augsburg in 1777 and wrote to his father Leopold that he now preferred Stein's instruments above all others, including his previous favourites made by Späth. In the letter, translated by Emily Anderson, Mozart praised the even tone of Stein's keys, the effectiveness of the knee lever, and Stein's method of deliberately cracking and regluing soundboards to ensure durability. Stein charged no less than three hundred gulden for these instruments.
Around 1790, Stein's daughter Nannette took over the firm when her father became too ill to continue. In 1794, she moved the business from Augsburg to Vienna with her husband Andreas Streicher and her brother Matthäus, continuing under the Streicher name. The firm remained in operation for over a century after that move, finally ceasing production in 1896.
In 1828, Friedrich Wieck acquired a Hammerflügel built by Stein's son André Stein for his daughter Clara (1819-1896), who later married Robert Schumann. That piano is preserved today in the Robert Schumann House in Zwickau and was depicted on the German DM 100 banknote issued in 1989.
About fifteen Hammerflügel bearing Stein's label survive, ranging in date from 1780 to 1794, of which two are privately owned (dated 1782 and 1783). Two clavichords also survive, including one now in the Budapest National Museum that was bought by Leopold Mozart. One instrument combining a piano with organ pipes survives in the Gothenburg Historical Museum.