Interesting about personal perspectives as always. KOB does convey a sense of mystery for me as well, but rather than melancholy I get a feeling of reflection. The word melancholy I relate to sadness while reflection is more of a sense of realization. Just the same, a very interesting point about Cannonball’s nature not fitting the mold feeling of the album, yet his contributions are (for me) indispensable. In all these years of listening I never ask myself about a change in Miles’ line up. ;^)
Considering Cannonball’s personality, he always seemed to me to find the fun in whatever music he played. Even a song with dour implications in the title like "Sack O Woe" may still be bluesy but not down -
https://www.youtube.com/watch?v=u6rU-4za3EQ