This is certainly a very difficult subject. Yeyasher Kochacha for giving me the opportunity of learning this deep Sugya.

1. It seems that the main purpose of the Chidushei Rav Nachum (page 202 to Yevamos 20a, #356) that you cited is to understand the opinion of Rebbi Shimon recorded in the Mishnah (18b). The Mishnah there discusses a case of two brothers, one of whom (Reuven) died without children. The second brother (Levi) did Yibum with Reuven's widow. Afterward, a third brother (Yehudah) was born, and then Levi also died. Rebbi Shimon maintains that when Reuven had two wives, Yehudah may now perform Yibum or Chalitzah with either one of Reuven's two widows. The Gemara (19b) explains that Rebbi Shimon maintains that since the moment from Yehudah's birth, he was permitted to the Yevamah and she was never forbidden to him for any moment, it follows that there is no prohibition of Eshes Ach she'Lo Hayah b'Olamo, since Rebbi Shimon maintains that this prohibition applies only when the Yevamah was forbidden to him at the time the new brother was in the world.

2. The Meiri (11a, DH Ravina) cites two opinions for how to understand the Shitah of Rebbi Shimon: (a) It is sufficient that Yehudah was born after Levi did Chalitzah. This is because the Chalitzah removes the "Zikah" between Yehudah and Reuven's wives and, consequently, the prohibition of Eshes Ach she'Lo Hayah b'Olamo (that Yehudah was not alive at the time that Reuven died and therefore cannot perform Yibum with his wife) does not apply. According to this, it makes no difference if Levi did Yibum with Reuven's widow or if he did Chalitzah. (b) It is necessary for Levi to have done Kidushin with the Yevamah after he did Chalitzah, and afterwards Yehudah is born. The reason is that since the Gemara (19b) states that Rebbi Shimon permits Yehudah because he "found her" when she was permitted, it follows that he finds her permitted only if Levi performed Yibum, while if Levi performed Chalitzah, Yehudah finds her when she is forbidden to him. The Meiri sides with the second way (b) of understanding.

3. Rav Nachum now attempts to explain the reasoning behind the second way of understanding. It will be noted that this way seems, at first glance, to be similar to the opinion of Rashi who writes above (11a, DH u'Metaretz) that Yehudah is permitted to the Yevamah only if Levi did both Chalitzah and Kidushin. (See Chidushei Rav Nachum earlier, 11a, #147, who explains Rashi's opinion at length.) However, Rav Nachum points out that there is a subtle difference between the explanation of Rashi and that of the Meiri.

4. To understand the difference, we must turn to the Gemara on 20a. The Gemara there asks a rhetorical question on Rebbi Shimon's opinion that since when Yehudah was born, the Yevamah was immediately permtted to him, and there was not a moment at which she was ever forbidden to him, there is no Isur of Eshes Ish she'Lo Hayah b'Olamo. The Gemara's question is that it should follow that if it so happened that the little Yehudah, in his mother's womb, had a sister from the same mother (but from a different father) who had married a brother of Yehudah's from the same father as Yehudah (but from a different mother), according to Rebbi Shimon after Yehudah is born why can he not do Yibum with this sister, since when Yehudah was born he found the sister permited to himself?

5. Rav Nachum (11a, #147) explains that when the Gemara asked this question, it thought that the reason why one is not allowed to perform Yibum with close relatives is that the prohibition against marrying those relatives existed before the Mitzvah of Yibum took effect, and therefore the Mitzvah of Yibum cannot override the prohibition. However, in a case where there was a Mitzvah to do Yibum first (before Yehudah was born), it should follow that the Isur against marrying one's sister is waived by the pre-existing Mitzvah to perform Yibum.

6. The above explanation, however, is valid only according to Rashi, who writes (20a, DH Achoso) that when the brother was born he found his sister with a Heter of Yibum. This means that the moment he was born, there was no problem (from the point of view of the Mitzvah of Yibum) in performing Yibum with his sister (aside from the general prohibiton against marrying one's sister). If we look closely, though, we will notice that Rashi adds one extra word which is not mentioned in the Gemara. The Gemara states that he found her with a " Heter ," but Rashi adds that he found her with a " Heter Yibum ." The Meiri does not add the word "Yibum."

7. Rav Nachum (20a, #356) writes further proof that the Meiri disagrees with Rashi who requires Heter Yibum. This is from a statement of the Meiri slightly earlier in the same piece (11a, DH Ravina). The Meiri explains that the reason why Yehudah is forbidden to Reuven's wife if, before Yehudah was born, Levi did Chalitzah, and then Yehudah was born and only afterwards Levi did Kidushin, is that Chalitzah is not a Kinyan but merely a "Petur," an exemption. (The source for this statement of the Meiri is the Yerushalmi here (1:1), cited also by the Chidushei ha'Rashba (19b, DH Hai Bava (#1) and footnote 102).) Rav Nachum points out that we learn from the Meiri that if Chalitzah would have been a Kinyan, Yehudah would have been permitted to Reuven's wife. Since Chalitzah is equivalent to divorce, because Chalitzah releases the Yevamah from Yibum, it follows that the Meiri does not require that Yehudah should find her with a Heter Yibum, unlike Rashi. Accordingly, we still have not found the reason why, according to the Meiri, it is considered that Yehudah found her when she was permitted only if there was Kidushin and divorce, and not Chalitzah alone.

8. Rav Nachum (#356, end of DH v'Nir'eh) concludes that the reason why Levi must do Kidushin is that we require that there be no connection whatsoever between little Yehudah and the deceased Reuven. It is therefore essential that the Yevamah should no longer be considered the wife of Reuven. This total break with Reuven is accomplished only if Levi does Kidushin with the Yevamah, because in this way she is called the wife of Levi and is no longer considered the wife of Reuven. Since she is not the wife of Reuven, it follows that there no longer exists on Yehudah the prohibition of Eshes Ach she'Lo Hayah b'Olamo, because Yehuda has no connection with Reuven any more.

Kol Tuv,

Dovid Bloom