Why must it say "v'Nikah ha'Ish me'Avon"? Why might we have thought otherwise?
Rashi: To reassure the husband that he will not be punished for bringing his wife to the Kohen and causing her death.
Ramban and Targum Yonasan (on Pasuk 21 DH 've'Zeh Ta'am', citing Sotah, 47b): What the Pasuk 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. 1
Rashbam: 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. 2
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. 3
Ramban: Or, as some commentaries explain, if he ever committed an act of Z'nus in his lifetime - even just once, the Mei Sotah would not work. According to the Gemara, even if his sons and daughters are 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 (when 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.
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.
What exactly does the Pasuk mean when it concludes "veha'Ishah ha'Hi Tisa Es Avonah"?
Rashbam: Refer to 5:31:1:3*.
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.
Moshav Zekeinim: This hints to three whose sins are pardoned - one who rises to authority (Mashi'ach Hashem v'Nikah - from his sins), a Choleh who healed (Im Yakum... v'Nikah), and a Chasan who marries; the Sotah died and her husband remarried, and via this he is pardoned.