Explain to me what a dominant function is? I don't get how every time a tri-tone exists within a chord it will be a dominant. This will clearly dismiss a nice range of chords. A leading tone only wants to do one thing (naturally) and that's move up a half-step but it doesn't have to. Example: viio - III7(V7) - vi(i). A deceptive move from major to harmonic minor a step and half lower. I get what your saying but don’t you think by shortening the range of chords into either I or V your limiting your possibilities?? Actually any chord is a shape of it’s tonic chord (I or i). So what help is it to think of every chord as a tonic if your a theorist?

I know why you don't agree (or see what I'm trying to show) cause you always make everything into another chord. Like your going to do with the chord progression I wrote above, change the III7 to a IM9(3rd in bass), which would no longer make that change a harmonic minor cadence. I see chord substitution as a way to create alternatives for chords, not a way to fit every chord into a smaller range of chords. A minor 6 chord is not a dominant 9 chord. I don't know how you are able to grasp the full use of music. In the world of sound, a minor 6 chord doesn't sound like a dominant 9th chord regardless of inversion. So they should not be thought of that way. 2 is better than 1, meaning if you distinguished them you would have 2 chords and more to create with.

You can't play a pure Ionian or Lydian scale over a dominant chord without a major clash. (unless you have deep experience in bebop and modern country.)

I disagree the 7th works as a good passing tone between b7-1. Looks like this b7-7-1. As a bass player I use that all the time. Also, a #4-5 will give you a nice blues trick, which is a popular blues chord. I'm not a fan of the natural 4th over any chord unless it's a suspension. Actually the only tones you want to be careful with over a dominant chord is the 4th, (b)9 (unless it’s a dominant (b)9th), and the #5 and 6.