Hello,
Interesting question that would require a full analysis of the harmony in the piece, (beginning very similar to prelude BWV847) here are some unsure points :
1) Layout of the work (x : X minor, X : X major):
1. Miserere I : c-v (v = dominant of c : g)
2. Miserere II : C
3. Gloria Patri I : e
4. Gloria Patri II : Cv-v (Cv = G, ends on dominant = D)
5. Sicut erat : C-v
6. Miserere II : c-v
2) The key has two flats (Dorian notation) as for G minor, although the C minor is the first established tonality. Two comments : first the theory of key notation wasn't fixed at that time, the number of accidents may vary. Also as G minor (dominant of C minor) is mainly used, the notation is more simple.
3) The main cadences are on 'us' of Deus : ms 24, 29 : in C minor.
4) In the bass, on most G, the figured bass includes the C in inverted chords.
5) It seems to be an exploitation of the harmonic modulation between tonic and dominant, giving an expansion/elevation effect, like a long dominant pedal. Used inside the movement, and between the pieces.
SK