Oomis,
The missing piece of the puzzle is that the Chiyuv to die is not directly connected to the punishment. According to the Rambam, if the person succumbs to fear and transgresses the Aveira he is considered an Oness and doesn’t get punished. Nevertheless, he was supposed to get killed rather than transgress.
The Halacha is that in these Aveiros, the branches of the Aveira are included in the Chiyuv.
Another thing is that you are mixing the concept of Oness and Mefuta in the case of a Naara Tehora with Nida, who’s marrital status doesn’t play a role.