The one place I'd disagree with you is on the positioning of Westerns; I think we recognize them much more by their setting-components than by their narrative ones. If you took away Firefly's accents and horses and dusty plains and guns that look like six-shooters and so on, it would just be an SF story about smugglers on the edge of the law, and while that's a common type of story to tell in a Western setting, I don't think it's definitional in the way the puzzle-solving of a mystery is, or the tropes of a Gothic novel.

As for terminology -- I, too, like the word "mode," after Brian Attebery's distinction between mode (fantastic vs. mimetic), genre (marketing category), and formula (specific narrative types -- the term is used non-pejoratively).

I wonder what accident of development classed alt-hist as science fiction? Its closest relative might be Ruritanian fantasy, but on the whole I tend to think of it as this weird third sibling in the family, who gets along well enough with the bickering duo of SF and F but doesn't really play the same games they do.