Definitely more than one way to tackle it. But I would think that eliminating ’ used as an apostrophe might be less tedious if done first. Once the ’ preceded and followed by a letter were eliminated, I would think a search for single closing smart quotes that
weren't followed by punctuation would cover the bulk of the special-case plural possessives, and ’Tis, and argot-like ’em and ’im (for them and him). There's always going to be the possibility that <span> tags might interfere with the detection of ’ followed by punctuation, but that's always going to be the case no matter how you tackle it. I would think, though, that getting a document into state where the ‘ and ’ represented only opening and closing dialog quotations wouldn't be an impossibly daunting task--if I was motivated enough to
want to do it.