Why does the Torah insert the phrase "ve'Nikah ha'Ish me'Avon"?
Rashi and Rashbam #1: To reassure the husband that he will not be punished for bringing his wife to the Kohen and causing her death.
Ramban #1 (in Pasuk 21, citing Sotah, 47b) and Targum Yonasan: What the Torah means is that the Mei Sotah will only take effect if the husband is himself free of sin, by not having been intimate with her from the time that she became a Sotah.
Ramban #2 (in Pasuk 21, citing Sotah, 47b). The Torah means that if he ever committed an act of Z'nus in his lifetime - even just once, 1 the Mei Sotah will not be effective. 2
Rashbam #2: The Pasuk is saying that the husband is absolved from the sin of remaining silent in face of his wife's infidelity, seeing as she became forbidden to him. In fact, had he remained silent, and continued living with her, he would have been guilty. 3
Seforno: It means that the Sotah's husband is absolved from the sin of having suspected his wife of having committed a sin which she did not commit. 4
Oznayim la'Torah, citing the Rambam: Even if he had relations with his betrothed wife - which is only Asur mi'de'Rabbanan. See Oznayim la'Torah who discusses the various opinions at great length.
Sotah, 47b: In fact, even if his sons and daughters were guilty of immoral behavior and he did not stop them, the Mei Sotah will not take effect. Because the entire issue of Mei Sotah is nothing but a miracle and a Kavod for Yisrael (provided they are worthy).
Rashbam: And, assuming that she committed adultery, it is his wife who must bear the entire brunt of the blame for her own death, as the Pasuk concludes. See also Torah Temimah, note 160, who explains that the various explanations do not necessarily disagree with one another.
Seforno: This is because the fault lies with her, for contravening his warning, thereby creating a situation whereby all the evidence points towards her guilt, as the Gemara writes in Shabbos, 56a, in connection with David ha'Melech..
Why does the Torah not add the word "ha'Ish ha'Hu" by the adulterer like it does by the Sotah?
Oznayim la'Torah: Because if she is guilty the curse will take effect on whichever man with whom the Sotah commited adultery, even if he is not the onee on whom her husband warned her. 1
What does the Pasuk mean when it concludes "veha'Ishah ha'Hi Tisa es Avonah"?
Rashbam: Refer to 5:31:1:4*.
Seforno: It means that, if she committed adultery, she will die, and if she didn't, she will be humiliated in front of all the people for having had the audacity to contravene her husband's warning.
Two other Pesukim use the term "ve'Nikah" - "Al Mish'anto ve'Nikah ha'Makeh" (Mishpatim Sh'mos, 21:19), and "Mi Shalach Yado bi'Meshi'ach Hashem ve'Nikah" (Shmuel 1 26:9). What do they have in common?
Moshav Zekeinim: They hint at three people whose sins are pardoned - one who rises to a position of authority ("Mashi'ach Hashem ve'Nikah" - from his sins), a sick person who is healed ("Im Yakum... ve'Nikah"), and a Chasan who marries; the Sotah died and her husband remarried, on account of which he is pardoned.
