For what it's worth, I think your whole post is just lovely! Perfect, really.
Thanks, Jim. It's worth a lot, actually. I always appreciate knowing when people like my posts. (When you're not working or in school, positive feedback, especially for intellectual matters, doesn't happen all that often.)
Feathers, I was speaking as potential "native English would-be teacher." I certainly agree with you and Jvm that understanding the distinction between the two types of problems is important. I'm just saying that the terminology applied to the distinction is arbitrary, and, at least if I were the student, it would be easier to focus on the inherent distinction if it was admitted from the beginning that the labels were arbitrary.
Actually, I learned a lot from this thread. Being only a casual student/teacher of language, I had never had the difference between the two categories of incorrect language use pointed out to me. I think it's interesting.