20. Is it true that Beethoven's mother got seven or more children, of which most of them were deaf or blind or otherwise disabled, probably due to syphilis?

No, that is not true. When she married Johann van Beethoven she was already a widow. Both her husband and her young baby had died. As far as I know nothing is known about the cause of their deaths. In 1769 she gave birth to Johann's first child, but it died at the age of six days. Then followed our great composer, then his brother Caspar Anton Carl, who died in 1815 of tuberculosis, then his brother Nikolaus Johann, who died in 1848 of old age. Three more children (one boy, two girls) followed, all of them died very soon, respectively at the age of four days, two years and one year. Nothing is known about the causes of their deaths. Beethoven's mother died of tuberculosis (in 1787), Beethoven's father probably of a heart attack (in 1792).

Литература:
Schmidt-Gorg, Joseph. Beethoven. Die Geschichte seiner Familie. (Bonn, 1964).
Schmidt-Gorg, Joseph. Des Backermeister Gottfried Fischer Aufzeichnungen uber Beethovens Jugend. (Bonn, 1971).
Mann, Werner. Beethoven in Bonn. Seine Familie, seine Lehrer und Freunde. (Bonn, omstreeks 1982).
Raab, Armin. Beethovens Mutter - Legenden und Tatsachen. In: Bonner Beethoven-Studien. (Bonn, 1999).