Yep Andrew, we agree.
I like superlocrian better for the dominant function in minor, as the Spanish (Or mixo b13) has that "snake charmer" interval between b6 and raised 7th, which can tend to sound a bit corny. Superlocrian just sounds more jazzy to me.
I like your concept of basing your scale choices on what notes work in relation to the key. If we are going to arrive at G minor, let's use as many notes from that key as possible on the preceding chord. Also, you've got a good idea to use the piano to work out harmony, and play what you like to hear.
C+9 generates a whole tone scale. Monk loves this sound on dominant chords.
Tonight I was teaching beginner theory at Vancouver Community College, and I pointed out how the dominant chord wants to pull us toward a tonic function. I played I, IV, V, and left the V hanging, so that they could feel the ear tugging toward the resolution on I. All these extensions and scales do is add more tension to the sound, pulling us more and more toward a tonic resolution. Tension and release, that's the ticket!
(I also like these chords and scales as worlds to themselves, ala Wayne Shorter's harmony.)