Nedarim 90b - Admiting to adultery

TO ME IT SEEMS THE CONNECTION (ALTHOUGH I AM NOT SURE WHY THERE HAS TO BE ONE) BETWEEN NEDARIM AND PURIM IS THAT THEY BOTH END WITH ADULTERY (BY ESTHER TO SAVE KLAL YISROEL).

In the Mishna Rishona a woman is believed to say she was raped (if she is a kohen's wife) or committed adultery intentionally (just that in the first case she would receive a kesuba and in the second she would not), to assur herself on her husband (someone asked why she is beleived according to the mishna rishona and we don't say ein adam meisim atzmo rasha - i think we discussed this issue in kesubos). In the Mishna Achrona we do not believe her because we are concerned she is lying to get out of the marriage.

The Ran has 3 explanations for this distinction: 1. Mei'ikar hadin she is believed, but the Rabonon have control to uproot a din d'oraysa and allow her to remain with her husband (and eat teruma). The Ran doesn't like this since he understands that the right of the Rabbonon to do this is limited to a temporary uprooting of a din b'kum v'aseh. The Ran is lishataso on 87a (by toch kdei dibur) where he rejects the same approach based on the fact that he holds that they can't do it permanently (the shita assumes that they can uproot for midgar milasa even permanently, and would be consistent with the first answer here). 2. Mei'ikar hadin she is believed but the rabbonon implement the fact that kiddushin is based on their da'as so they have power to render her "unmarried" at the time of the rape or adultery. Based on this approach they would presumably assume that when she lives with her husband after that time, it creates a new marriage and that is how we allow her to eat teruma. 3. In this approach the Ran assumes that whether or not she is believed is entirely based on whether chazal assumed based on an umdana she is telling the truth (mishna rishona bec. of embarrassment), or that she is lying (mishna achrona bec. of wanting to marry someone else). Based on this we understand why she can eat Teruma.

This is all regarding her level of believability. But there is still the issue of shavya anafsha chaticha d'issura (which is either m'din neder or like a hoda'as ba'al din) that creates an issur on her. Tosafos quotes a famous answer from Rabbeinu Eliezer that an eishes kohen who is raped and assur as a zonah is only an issur on the kohen, not on her, so since she is not believed, she is allowed to remain with him (there is still a question of how she can be machshil him to do an issur). Tosafos offers another answer that chazal have the right to take away her ne'emanus and remove her ability to create a shavaya anafsha chaticha d'issura.

Now, if an eishes kohein would be raped and claim she was raped, the beis din doesn't beleive her so we don't force divorce. But, when she asks her private Rav whether she is allowed to go back to live with him, the halacha would be that it is assur for her to live with him if she knows that she was actually raped. According to Rabbeinu Eliezer in Tosafos where there is no issur on her, she can live with him. But according to most rishonim who reject this chiddush, her Rav would have to tell her she must become a moredes and refuse to go back (even though we won't force him to give kesuba or a get). If a woman claims that she committed adultery willingly, all would agree that even though she is not believed in beis din. it is assur for her to go back home since she knows that she was mezaneh.