I have little to add to this lively discussion except to say that recently
I & others also struggled w/ the issue of what to call
the-concept-previously-known-as-grammar (maybe we could just use a symbol,
like Prince does).
 
I was working w/ my co-author & editors in coming up w/ a subtitle for our
textbook (Now I Get It), and after several discussions we went w/ the
subtitle A Commonsense Guide to Grammar & Usage.  We decided not to use the
term "conventions" since that's not a conventional term for the concept,
"mechanics" (my original choice) seemed too vague & dull, and "punctuation"
was too exclusive.
 
So we went w/ 2 terms (grammar & usage) that seemed to convey the idea of
the book's content to a large audience, though "usage" for me still seems
to deal with word choice rather than things like run-on sentences.
 
Our lack of an appropriate, widely-acceptable name for this "thing" is
probably a reflection of (1) how our professional community & professional
organizations have neglected the subject and (2) how complex this
seemingly-simple subject is.
 
Larry Beason,Director
English Composition Program
Dept. of English
Eastern Washington University
Cheney WA 99004
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WAC Page: http://ewu66649.ewu.edu/WAC.html