Baz, it seems to me that the correct time signature is in fact 12/8. I'm sure someone must have recorded and/or performed this version, or something akin to it. As for practicability, I don't see why it couldn't work at around dotted quarter=66.
"12/8" can't be right! In that signature, each 8th note would divide into only 2 16ths. But here the groupings are in 3s. "24/16" is technically correct, but stupid - i.e. the unit of time is not the shortest note of the group, but the longest! What is wrong with Bach's own signature anyway?
In "alla breve" notation (used in both versions by the composer) both the cut-circle
and the cut-semicircle mean the same thing -
2/2. The beat is therefore shifted to the half note (minim), indicating a brisker tempo in which (under normal conditions) the "flow" of the music is projected upon a canvas consisting of quavers rather than semiquavers. BUT in both versions, this procedure is given the added pace of a background movement consisting also of semiquavers. I should maintain, therefore, that the tempo is supposed to be breathtakingly
quick.
Under such conditions, fitting in all the rhythms is easy (assuming a sufficient technical ability to play the piece at the required tempo), and doesn't necessarily call for
absolute precision in the placing of the shorter notes/rhythms against each other (provided they all fit into the overall unit of tempo). For this piece, I should suggest the slowest viable tempo to be two beats per bar at 60 (though if a little quicker that would be more effective).
This kind of complexity is quite common in Bach - and usually in the end is resolved by adopting a suitable overall tempo. Another good example is the chorale setting of
Vater unser from
Clavierubung 3 (BWV 682). Here is the ending of the piece (as it appears in the original print), showing how the beats variously divide into 3s and 4s. Some help is given by the print in resolving performance issues (as indicated by the alignments that I have placed in red rectangles). But the main issue is the tempo (which is usually far too slow in performances): unless it
flows the canonically treated chorale melody (between alto and tenor) becomes difficult to follow, and the complexity of the smaller note groups comes to the forefront rather than providing the background canvas.
Baz