I was going to let this piece go, but H. S. Teoh's objection that the ending C major sounded like the dominant of F has haunted me, and eventually prompted a major revision of the penultimate section before the Coda. I am still considering this version "experimental", but I very much like the result - the last fugal section is now made to cadence in C minor and the Coda begins in that key. Although this was done basically by transposition, there are several changes of register in the various parts and the counterpoint now becomes quite florid at its highest point, with the lines crossing and re-crossing. Also, the score now features the various "choirs" (string quartet, chamber strings, full string orchestra) written out explicitly, which allowed me to separate them spatially in the rendering (Sibelius + NotePerformer), making it easier for the listener to tell them apart.
Audio file and draft score offered for your enjoyment, though any feedback would be welcome.