Number of beats doesn't match time signature - how to play this?

Question

I have a question about sheet music for D. Scarlatti's sonata K56. The time signature of the piece is 12/8, but in the very first measure there are actually 12.5 beats in the upper staff, including the 1/8 rest at the beginning. In the lower staff there are 12 beats as expected.

My question is, how this should be properly played. There are basically 2 possible solutions:

1. Reduce the duration of the rest at the beginning from 1/8 to 1/16. This would bring the number of beats to the desired number of 12. However, this would also create a partial overlap between the C3 and C5 notes, which was maybe not intention of the composer.
2. Make the first measure slightly longer. This would be equivalent to adding a 1/16 rest to the lower staff.

Answer 1

Each bar (measure) would be the same length time-wise. Like it is in almost every piece.

Your calculations have been thrown because there is a tuplet in the first bar. The bracket with a number 5 is the culprit. Those five notes need to be played in the time of two quavers. That, along with the quaver rest, will account for the first three quavers in 12/8. If you prefer to count in a slower four, each of the three quavers counts one. The first count of one, in that case, is a short rest, then all five notes, to land on beat two (the A♭).

Answer 2

Actually, there isn't 12 beats - it's 4. A compound 4, indeed (3 eight notes for each beat).

First beat: first 8th is rest, the other 2 are subdivided into a 16th quintuple (5 notes in the place of 4). No problem here at all, just divide this time frame in 5 equal parts.

The second, third and fourth beats are ok too, 3 8th notes each. In the next measure, this figuration is repeated, but with regular 16th notes now.