Here is the Breitkopf Complete Works edition of the Sonatas: https://s9.imslp.org/files/imglnks/usim ... _Op_65.pdf
There is an earlier Breitkopf score which seems more "keyboardistic" in its notation (with Mendelssohn freely using the top two staves as a grand stave similarly to how piano composers used to), but only the first and sixth sonatas are available on IMSLP.
I wanted to point out some of the moments that show the intentionality and the thoughtfulness behind Mendelssohn's markings, in my eyes:
At the beginning of the Third Sonata, there are slurred pairs in the manuals against a non legato bassline, which helps to keep the very heavy pedal line incisive and clear. As the march gets moving, the pairs gradually transform to groups of four slurred notes:
During the dialogue between first and second manuals, he writes no slurs to give the march a more pompous character, contrasting with the more vocal solo line.
The two halves of the fugato that follows are written with differing articulation. But when the pedal line plays a chorale tune against the fugato, it is specified as non legato:
When the pedal does finally get the subject itself, though, it is articulated precisely as in the hands.
This all seems to be lost in the recordings I listened to.
The second movement starts with a delightful ambiguity of metre, through articulation and harmony, that is only resolved in the fourth and fifth measures.
The final movement of the Fourth Sonata starts with a long slurred line in the pedals, leaving the player more comfortable to strongly articulate the two-note figure in the hands:
A little later, this wonderful contrast of articulation occurs, and the two articulated notes in a row are a delightful "snap" in between the long slurred lines, feeling very physically satisfying.
The Sixth Sonata (variations on Vater unser im Himmelreich) begins with the chorale tune non legato, which lends a foreboding, almost impersonal air to the passage:
In the first variation, which is very quiet and light, he slurs all of the pedal offbeat entries without question.
The final brillante variation mixes "ordinary" passagework with ascending arpeggios marked with slurs that I am convinced have to be played "pianistically", i.e. held over until the full chord is present, with the following block chords played almost as an echo and not that sharply. When Mendelssohn does not want it (at the very end of this excerpt), he marks both chords with a dot (the English first edition has wedges).
I am saddened that this detail, which gives the pieces so much vitality and freshness, seems to be glossed over by most organists, even as keyboardists of all stripes are adding much more detail in articulation to earlier music.
