The Scarlet Pimpernel (novel)/Headscratchers

Did I miss some Xanatos Gambit in The Triumph of the Scarlet Pimpernel?
Chauvelin's appearance at the fortuneteller's in the beginning suggests that using Theresias as a Honey Trap was originally his idea, but seriously thinking any woman could successfully seduce the Scarlet Pimpernel seems too Genre Blind even for Chauvelin! True, he implemented a different plan and used Theresias in a different way after she realized such a plan was impossible, but did he expect her to initially fail?
 * He could have been playing Xanatos Speed Chess.

We could have all avoided this in Eldorado
While reading Eldorado for the first time, when I reached Part II, I initially wondered why Chauvelin didn't pull an I Have Your Wife with Marguerite like in The Elusive Pimpernel and try to force Sir Percy to give up his information that way. I then actually considered, "Maybe he's actually learned his lesson that that never works"... until Part III, where he goes and holds Marguerite hostage to force her husband's hand! If, in context, Chauvelin sees no problem with using this plan again, why didn't he just do that in the first place as soon as he located Marguerite in Paris instead of waiting another 10 days? Is there any way to explain this besides Genre Blindness?