Short answers, pulled from the story.
Muzio Clementi (1752-1832) was an Italian composer, pianist, publisher, and piano manufacturer who spent most of his career in England. He earned the title "Father of the Piano" because he was among the first composers to create keyboard works expressly for the capabilities of the piano rather than the harpsichord, and he developed the legato technique that defined classical piano playing for generations.
On the 24th of December 1781, Clementi and Mozart performed for Joseph II, Holy Roman Emperor, in a musical contest at the Viennese court. The Emperor declared a tie. Mozart privately dismissed Clementi as "a mere mechanic" with "not a kreuzer's worth of taste or feeling," while Clementi recalled being overwhelmed by Mozart's spirit and grace.
Clementi's students included Johann Baptist Cramer, Ignaz Moscheles, Johann Nepomuk Hummel, John Field, and Ludwig Berger, who went on to teach Felix Mendelssohn. John Field in turn became a major influence on Frédéric Chopin, extending Clementi's pedagogical impact across the entire 19th century.
Beethoven had an exceptionally high regard for Clementi's sonatas. His assistant Anton Schindler recorded that Beethoven considered them "the most beautiful, the most pianistic of works" and confined the musical education of his nephew Karl to playing them for years. Beethoven reportedly said that those who studied Clementi thoroughly simultaneously learned Mozart and other composers, but that the reverse was not true.
Clementi took over the London firm Longman and Broderip at 26 Cheapside in 1798 and became both a music publisher and piano manufacturer. In 1807 a fire destroyed his warehouses on Rotten Road at a loss of about £40,000. That same year he secured publishing rights to all of Beethoven's music in England. He also co-founded the Philharmonic Society of London on the 24th of January 1813, which became the Royal Philharmonic Society in 1912.
Gradus ad Parnassum is a collection of keyboard studies that Clementi completed in 1826. He intended to publish its third volume simultaneously in Paris, London, and Leipzig. The work became a standard pedagogical text; Chopin required his own pupils to practise Clementi's preludes and exercises throughout his teaching career.