In your insights in the Daf, you quoted Rashi's opinion that the Mitzvah to be Poresh from a Nidah refers to Hafrashah Samuch l'Vestah. But our Mishna implies that the Aseh she'b'Nidah has a Chiyuv Kares. Even if Hafrashah Samuch l'Vestah is D'oraysa, no one said it has a Kares.
According to the other explanations of l'Frosh b'Eiver Meis the Sugya in Shevuos implies that there is not an Isur Kares for example, the distinction the Gemorra (18) makes between Anud and not Anus.
Furthermore, even if Poresh b'Eiver Chai is Chayav Kares what act did he do to be Chayav.
Obviously, in order for the Tzibur to bring a Korban there must be a Chiyuv Kares for the Isur when done b'Mezid. This is the difficulty with Shitas Rashi. Seemingly Rashi will learn that the outcome was Kares -- i.e. there was no Perishah b'Sha'as Veses, and the woman ended up seeing b'Sha'as Tashmish. In this case, Rashi will hold that the Hora'ah caused an Isur Kares.