Gemara

(Mishnah): If a Sotah's husband dies, she does Chalitzah, not Yibum.

Question: Why can't she do Yibum?

Version #1 - Answer #1 (Rav Yosef): "She will leave his house and be l'Ish Acher (to another man)" - to another, not to the Yavam.

Question (Abaye): If so, she should not even need Chalitzah!

Answer (Rav Yosef): If the husband were alive, she would need a Get (document of divorce). Now that the husband is dead, she needs Chalitzah.

Version #2 - Answer #1 (Rav Yosef): "She will leave... l'Ish Acher ", lest she wreck her (first) husband's house. You cannot say that she will do Yibum!

Version #3 - Answer #1 (Rav Yosef): Her next husband is called "Acher", i.e. unlike the first husband, who expelled evil from his home. The second one brought evil into his home! You cannot say that the Torah commands the Yavam do to Yibum! (end of Version #3)

Answer #2 (to question 3:b on Daf 5b - Rava): A Kal v'Chomer forbids Yibum. If she became Asur (forbidden) to the one she was permitted to (i.e. her husband), all the more so she becomes Asur to the one she was Asur to (the Yavam)!

Yevamos 11a (Rav): The Tzarah of a Sotah is forbidden. She (a Sotah) is called Tamei, like Arayos.

Question (Rav Ashi - Mishnah): If she was secluded long enough for Bi'ah, and her husband dies, she does Chalitzah but not Yibum. (Rav exempts even her Tzarah from Chalitzah!)

Answer: Rav discusses a Vadai Sotah (she definitely had relations). The Mishnah discusses a Safek Sotah.

Question: Does he say that a Vadai Sotah is different because she is called Tamei? Also a Safek Sotah is called Tamei!

(Beraisa - R. Yosi ben Kipar): If a divorcee remarried (and was widowed or divorced), she is Asur to her first husband. If she had only Kidushin with her second husband, she is permitted to her first - "Acharei Asher Hutama'ah";

Chachamim forbid in both cases. "Acharei Asher Hutama'ah" refers to a Sotah who was secluded.

Answer: 'Secluded' is a euphemism for Zenus. (A Safek Sotah is not called Tamei).

Rishonim

Rambam (Hilchos Yibum 6:19): If there are witnesses that Ploni's wife was Mezanah b'Mezid, and he died before divorcing her and she fell to Yibum, she and her Tzarah are exempt from Chalitzah and Yibum. It is as if she was Ervah to the Yavam, for it says Tum'ah - "v'Hi Nitme'ah. If the husband of a (Safek) Sotah died before making her drink, or if she may not drink, rather, must be divorced, she does Chalitzah, not Yibum. Her Tzarah may do Chalitzah or Yibum.

Rosh (Yevamos 1:4): Rav teaches that Tzaras Sotah is forbidden, i.e. exempt from Chalitzah. This is clear from the question and answer from Safek Sotah, who needs Chalitzah. It says Tum'ah about a Safek Sotah, i.e. to be stringent to forbid her due to Safek Tum'ah. We are not lenient to exempt her from Chalitzah.

Question: Why did the Gemara (Sotah 5b) ask why a Sotah may not do Yibum, and struggle to answer? Yibum should be forbidden due to Safek Tum'ah!

Answer (Rosh): A Lav of Tum'ah is written only regarding Sotah Vadai. A Safek Sotah is not considered even like Safek Ervah to forbid Yibum. Indeed, the Gemara initially thought that the Aseh of Tum'ah should make her like Ervah. We answered that only a Lav makes her like Ervah.

Rif (Yevamos 2a): Rav forbids the Tzarah of a Sotah. A Sotah is called Tamei, like Arayos. A Mishnah teaches that if she was secluded, she does Chalitzah but not Yibum. This refers to a Safek Sotah. Rav discusses a Vadai Sotah

Rebuttal (Ra'avad, cited in Sefer ha'Zechus): We said that if her husband were alive, she would need a Get. Now that the husband is dead, she needs Chalitzah!

Defense #1 (Magid Mishneh 6:19): Perhaps the Sugya in Sotah is like Rav Yosef, who challenged Rav. We follow the Sugya in the Masechta that primarily discusses the question (Yevamos).

Gra (EH 173:12): 'Tum'ah' teaches that her Tzarah is like her. Rav needs "l'Acher", not to the Yavam to exempt the Sotah from Yibum.

Defense #2 (Rosh): The Ra'avad equates a Safek Sotah with a Vadai. He holds that the Sugyos argue; he is stringent like the Sugya in Sotah. Really, the Sugyos do not argue. A Safek Sotah needs Chalitzah; a Vadai Sotah does not.

Rebuttal #1 (of Rosh - Lechem Mishneh 6:19): The Ra'avad learns from Sotah 5b, which considers even a Safek Sotah like Ervah. We distinguished Safek from Vadai only to answer for Rav, who exempts even from Chalitzah.

Rebuttal #2 (Hagahos ha'Gra): In Sotah we learn from "v'Yotzah...", which is written regarding a Vadai Sotah! Also, the need for a Get obligates Chalitzah (and this applies also to a Safek Sotah)!

Hagahah: Perhaps the Rosh holds like Tosfos (below).

Tosfos (Sotah 5b DH l'Acher (2)): How can we learn about a Safek Sotah from "l'Acher", which applies to a Vadai Sotah? We must say that "Tum'ah" teaches about a Vadai Sotah, so "l'Acher" is free to teach about a Safek Sotah.

Poskim

Shulchan Aruch (EH 173:11): If witnesses say that Ploni's wife was Mezanah b'Mezid, and he died childless, she is like Ervah. She and her Tzaros are exempt from Chalitzah and Yibum. If it is a Safek whether or not she was Mezanah, she does Chalitzah, not Yibum. Her Tzarah may do Chalitzah or Yibum.

Gra (15): If it is clear that she was Mezanah, this is like witnesses.

Beis Shmuel (11): She is Asur to the Yavam only if she was Asur to her husband.

Even Pinah (cited in Hagahos Ma'aseh Nisim in Shulchan Aruch ha'Shalem): He excludes if her husband does not want to make her drink, he chooses to forbid her to himself, so she is permitted to the Yavam.

Rema: Some require Chalitzah even if she was Vadai Mezanah; the same applies to her Tzarah. One should be stringent.

Gra (13): The Ra'avad's opinion is primary. Yevamos 3b says that the Mishnah teaches unlike Rav. This shows that the Halachah does not follow Rav.