Short answers, pulled from the story.
Anton Walter (1752-1826) was a German-born piano builder working in Vienna, described by the Grove Dictionary of Music and Musicians as "the most famous Viennese piano maker of his time." Haydn, Mozart, Beethoven, and Schubert all owned and played on his instruments.
Walter added a back check to the piano action, a component that catches the hammer on its descent and prevents it from bouncing during lively playing. This innovation was adopted by other Viennese makers during his lifetime and remains standard in modern pianos.
Yes. Wolfgang Amadeus Mozart bought a Walter piano in about 1782 and used it during the composition and premieres of his mature piano concertos. The instrument survives today in Salzburg, though it was considerably modified by the Walter firm around 1800, nine years after Mozart's death.
Surviving Walter pianos are held in institutions across Europe, including the Germanisches Nationalmuseum in Nuremberg, the Musical Instrument Museum in Poznan, the Musikinstrumenten-Museum in Berlin, the Württembergisches Landesmuseum in Stuttgart, the National Museum in Prague, and the Richard Burnett Collection in England, among others.
Walter ran a successful workshop in Vienna that by 1800 employed about 20 workmen. His stepson Joseph Schöffstoss joined the firm in 1800, after which pianos were labeled "Anton Walter und Sohn." The last surviving piano from the shop is dated 1825; Walter died in 1826.
Philip Belt, Chris Maene, Paul McNulty, Paul Poletti, and Rodney Regier are among the modern builders who have constructed replicas of Walter's instruments for historically informed performance. These replicas have been used in recordings by performers including Malcolm Bilson, Kristian Bezuidenhout, Paul Badura-Skoda, and Viviana Sofronitsky.