I’m not actually attacking your point about the “nonsensical doubting” being the flagship. I’m saying that the title On Certainty focuses on the wrong thing.
The “star” of the book imo isn’t the certainty itself; it’s the misapplication of doubt. Wittgenstein’s interest is with certainty as the limit where any normal speaker’s “spade is turned.”
Calling it On Certainty treats the “limit” as a destination. Calling it On Doubt treats the book as what it is: a diagnostic of the “idle wheel” of Cartesian skepticism. It’s not a manual on how to be sure; it’s a study of why the skeptic’s spade cannot dig further and remain grammatically sound.