One course won't do it. I'm thinking more along the lines of a GRE-type test. A serious certification system with measurable (real) criteria. Problem with this is that there are how many different Style books out now? And then there are hybrid editorial guidelines for particular publishers. Not to mention that grammar and punctuation are really only a tip of the iceberg of serious editorship. Diction and syntax can be correct numerous ways, but a really good editor (text/line/copy/story editor) can massage it into the best it can be. A great one can show writers how to do that for themselves.
And then there's acquisitions. And editorial correspondences. And editorial writing. And article writing. And . . . and . . . and
I don't think that's at all a valid measure of skill or talent. Readership is more a measure of marketing than it is writing or editing.
Not to say that it is not one of the highest goals, just to say that it has nothing to do with real credibility. Statistics . . . are what you make of them.