14. Are Beethoven's famous metronome markings reliable or not?

Yes. Notwithstanding all the noise about them over the years there's actually not the slightest trace of a proof that Beethoven had misread his metronome or had suffered from a "manic-depressive attack" or had been too deaf (the markings date from 1817 and later) or had used a disabled metronome or had erred in whatever what other way. It's not difficult to devote many a book to all the attempts to "explain" his "unplayable" markings. In the seventies of the 20th century it finally turned out that Schindler, his first (and very unreliable) biographer, is the source of all the confusion. Schindler disliked Beethoven's metronome markings. He considered them far too fast. After Beethoven's death he embarked on a enterprise that was to become -maybe- the main goal of his life: the "correction" of those markings into a much slower direction. With this goal in mind he "rewrote" parts of the conversation books and even produced a fake score: WoO.162. For many a decade performers took Schindler seriously and overlooked his many forgeries (not only on the metronome markings, but also connected to various important biographical problems, alas). Thanks to the research of the German researchers Beck and Herre and the American researcher Howell we now know that it's high time to throw all the speculations about Beethoven's so-called unplayable metronome markings into the dustbin. They are playable indeed. However, it needs a small, but first-class orchestra, a first-class conductor and first-class soloist(s). There is only one exception and unfortunately this is Beethoven's most famous composition: his last symphony. When he wrote down the metronome markings for this music his personal circumstances were difficult and careful research has shown that he obviously made a few errors. The discussion on this complex problem is still going on.

Stadlen, Peter. Beethoven and the metronome. In: Music and Letters (1967).
Dohl, Friedhelm. Beethoven '77. (Zurich, 1979).
Beck, Dagmar und G.Herre. Einige Zweifel an der Uberlieferung der Konversationshefte. In: Bericht uber den Internationalen Beethoven-Kongress Berlin 1977 (Leipzig, 1978).
Seifert, Herbert. Beethovens Metronomiserungen und der Praxis. In: Beethoven-Kolloquium 1977 (Kassel, 1978).
Metzger, Heinz-Klaus und R.Riehn. Beethoven/Das Problem der Interpretation. In: Musik-Konzepte 8 (Munchen, 1979).
Beck, Dagmar und G.Herre. Anton Schindlers fingierte Eintragungen in den Konversationsheften. In: Zu Beethoven. Aufsatze und Dokumente (Berlin, 1980).
John, Kathryn. Das Allegretto-Thema in op.93, auf seine Skizzen befragt. In: Zu Beethoven. Aufsatze und Dokumente 2 (Berlin, 1984).
Howell, Standley. Der Malzelkanon - eine weitere Falschung Schindlers? In: Zu Beethoven. Aufsatze und Dokumente 2 (Berlin, 1984).
Goldschmidt, Harry. "Und wenn Beethoven selbst kame..." Weitere Aspekte zum Malzelkanon. In: Zu Beethoven. Aufsatze und Dokumente 2 (Berlin, 1984).
Eichhorn, Andreas. Beethovens Neunte Symphonie. (Bonn, 1986).
Kolisch, Rudolf. Tempo und Character in Beethovens Musik. (Munchen, 1992).