All right: As so often is the case, your questions have shown me the errors of my ways. Here’s what you’ve helped me learn:
Here’s what I’ve learned—some of which contradicts my statements above:
With Substitutions on for Smart Quotes …
TextEdit appropriately handles longer headlines that begin with a single quote followed by an initial word that begins with N, but it does reverse the quote for a single letter N. (This is appropriate behavior—especially since it can be manually overridden.)
Safari doesn’t work that way. Any single quote before an N (a single letter or a whole word) always turns into an apostrophe. Can be overridden.
Other browsers, including Edge and Chrome, apparently DON’T recognize Substitutions on quotation marks. (But they do recognize other manually entered replacements.) The smart quotes I’ve been getting are apparently generated there by extensions I’d thought weren’t working—like Instant Smart Quotes. But the behavior there is the same—the reason I posted this question in the first place: Can’t have a single opening quote before an N or a word beginning with N—and can’t be overridden.
If you’re confused now, join the club. And thanks again for your probing questions, which have helped bring the glitch into focus.