The Lessons Not Learned.

[Shurrikhan]

The correct address would be 石川さん or, even more formally, 石川様.

Again, though, that would not necessarily be correct, and adding 様 in any such case (she's not the head of some conglomerate that you work for, nor simultaneously a person of broad/ubiquitously familiar social prestige) is only going to come off as cultishly sycophantic. The titles with which you might address a person in Japanese do not necessarily correspond to the titles with which you would refer to that person.

There's a reason the JP interviews, unless written less formally and by interviewers primarily describing the social experience of talking with Yoshida, will never include さん in the actual exposition or summaries, only in the direct quotes of the questions addressed to Yoshida, about Yoshida (to his coworkers), or about his coworkers (to Yoshida), etc.

There's a reason, also, why the meme Daddy of Light comparisons choose deliberately, not as a matter of social course, to append さん to his name: it both mirrors 叔父さん, as per the show, and treats him as a more familiar figure, akin to a neighborhood dad. Such is specifically less formal and, outside of such niche situations, somewhere between less polite and outright odd by making the person in question seem as acquaintance despite their not being your --nor your listeners'-- acquaintance.

It does not share a one-to-one symmetry of use with English titles (just like not every nation's former head of state may still go by "Mr. President" / "Prime Minister" / etc.); there's more nuance to it and extant rhetorical appeals that would made impossible if one were to just apply those title in all cases.