Dear Stormx,
1) The left hand in bar 13 is in octaves, which helped to create the dynamic difference on Mr. Bach's piano.
2) D, without a doubt. The harmony is dominant seventh, and there is already a C on beat 4, which resolves to the Bb on the downbeat of the following measure (the D in question resolves down by 5th/12th).
Best wishes,
Jean-Baptiste Morel