Short answers, pulled from the story.
The Neue Mozart-Ausgabe is a scholarly critical edition of all of Wolfgang Amadeus Mozart's compositions, published between 1956 and 2007 by Bärenreiter-Verlag. It consists of 132 volumes containing 25,000 pages of music, arranged in ten series and 35 work groups, accompanied by 8,000 pages of critical commentary.
The Alte Mozart-Ausgabe was the earlier complete edition of Mozart's music, published by Breitkopf & Härtel between 1877 and 1883 with supplements until 1910. The Neue Mozart-Ausgabe replaced it with a historical-critical approach, incorporating the latest philological-musicological methods alongside practical knowledge of performance.
The largest single collection of Mozart autographs, held by the Berlin State Library, was removed during World War II and remained unavailable to NMA editors until 1980. This forced some editors to work from incomplete sources or pre-war photographs of manuscripts.
The pianist Rudolf Serkin provided a pre-war photographic reproduction of the autograph score for Piano Concerto No. 27, K. 595. Editor Wolfgang Rehm relied on this photograph in 1960 because the original autograph was inaccessible at the time of publication.
Frederick Neumann, in his book Ornamentation and Improvisation in Mozart, found some of the NMA's ornamentation suggestions misjudged, particularly regarding vocal cadenzas. Neumann also acknowledged in his preface that, where he could not consult primary sources himself, the NMA "offered an indispensable supplement."
Scans of the vast majority of the Neue Mozart-Ausgabe became available online on the 12th of December 2006 as the Digital Mozart Edition (DME), offered in JPEG and PDF format. The project was organized by the International Mozarteum Foundation in cooperation with the Packard Humanities Institute.