Connie,
     You say that there is evidence that "a systematic" approach doesn't
work, but WHICH systematic approach are you speaking of? There is a
major difference between a systematic approach that attempts to teach
grammar, and a systematic approach that attempts to teach students how
to USE grammar to analyze what they read and write. You asked what I
consider success. Ideal success would be to enable every student who
graduates from high school to be able to explain how any word in any
sentence syntactically relates to the main S/V pattern. That's what the
KISS approach attempts to do. If you have looked at TGLA, you will have
noted that I also discuss how the approach enables students to see
connections, not just with errors, but with vocabulary, style, reading,
writing, and logic.
      To be honest, I haven't read your Teaching Grammar in Context. I
heard that it was close to your Grammar for Teachers, which I did read.
It didn't take me long to come to the conclusion that your approach will
not work. The conclusion is based on the fact that almost all the
sentences you use for examples are extremely simple -- they do not
reflect the sentences that students generally read or write. (Mellon,
years ago,  noted this problem with most grammar books.) No matter how
systematically you teach it, it is not going to help students much
because it does not address their basic problem. Most errors result as a
by-product of syntactic growth, and the students' problem is in
untangling the deeply embedded structures (clauses within clauses, etc.)

     If I am wrong about your new book, and it does address teaching
students how to untangle the complexities in their own writing, please
let me know. I'll get a copy and read it. As it stands, however, you
can't seem to make up your mind -- you are for a systematic approach,
you are against a systematic approach.
      While I'm on this topic, let me note that we should never expect
students (or even adults) to always produce error-free writing. In
addition to simple performance errors, we all have problems with errors
as we try to get more and more ideas into single sentences.
Comma-splices, run-ons and fragments blossom in seventh and eighth grade
because that is when students are mastering subordinate clauses. I have
suggested that we should ignore these errors in seventh and eighth
grades. By focussing on them, we may simply be reinforcing them. If,
instead of focussing on errors, we teach students to analyze ANY
sentence, then the students will see how sentences work. Once they see
that, they will understand for themselves what is wrong with
comma-splices, etc.
Ed V.