Thank you for your very interesting and important question.

(a) Rav Moshe Feinstein zt'l (in Igros Moshe YD 3:138) himself explains why the Halachah follows Ravina. Ravina is chronologically the last of the five opinions mentioned in the Gemara in Sanhedrin 110b. (See Bava Metzia, beginning of 86a, which says that Rav Ashi and Ravina constitute the end of the instruction of the Talmud. In other words, Ravina is the last of the Poskim of the Gemara.) Rav Moshe also hints at a Halachic principle that states that one follows the later opinion among the Amora'im. (See Rosh, Bava Metzia 1:49, who says that from Abaye and Rava onwards, the Halachah follows the later Amora'im, and see also Rashi to Nidah, 7b DH Ha, that the later Amora'im were more precise and particular than the earlier Amora'im in fixing the Halachah, which is why the Halachah follows the later opinions.) This is why Rav Moshe writes that the Halachah follows Ravina.

(b) This is a very perceptive comment about the different opinions in the Shulchan Aruch concerning when the child is Zocheh to Olam ha'Ba. I would like to suggest an approach which I believe does not contradict Rav Moshe, but can explain how we find different landmarks mentioned in different places for when the child merits Olam ha'Ba.

1. The opinions in the Gemara do not necessarily disagree with each other, but rather they represent different stages in the spiritual development of the child. At each new stage that the child reaches, he is Zocheh to a higher level of Olam ha'Ba. Therefore, in OC 124 -- where the subject under discussion is that of answering "Amen" -- the Rema writes that one should teach his young children to answer "Amen" because when they do so they reach a higher level of Olam ha'Ba.

2. In contrast, the Shulchan Aruch in YD 263 is discussing Milah, so he therefore says that the deceased baby should be given a Bris even before he has reached the age at which he would have been able to answer "Amen" had he lived. Even though he did not reach the higher spiritual level of saying "Amen," nevertheless he did reach the stage of Milah, and therefore he can be Zocheh to this level of Olam ha'Ba. Again, there is no dispute in the Gemara between the different opinions, but rather each one is pointing out a different and new level of Olam ha'Ba.

3. One might challenge my argument on the grounds that Rav Moshe writes that the Halachah follows Ravina, which suggests that there really *is* a Halachic dispute in the Gemara, and the different opinions actually *do* disagree with each other. However, I would point out an interesting thing about the Gemara in Sanhedrin. Although we have noted that Ravina is the latest opinion chronologically, it is also conspicuos that his stage of the development of the child is the earliest of all. So when Rav Moshe writes that the Halachah follows Ravina, he is telling us that even from the extremely early stage of conception, the child already is Zocheh to Olam ha'Ba. The context of Rav Moshe's Teshuvah there is to show to his questioner that his brother, who died as a stillborn baby, still will be Zocheh to arise at Techiyas ha'Mesim -- because the Halachah follows Ravina that even if the baby was merely conceived and no more, he will reach Olam ha'Ba. Surely, though, another child who received a Bris Milah or said "Amen" will be Zocheh to Olam ha'Ba.

Kol Tuv,

Dovid Bloom

Here is a different and simple approach to explain the different opinions cited in the Shulchan Aruch. It may be that there indeed is a dispute in the Gemara about the stage at which a child can attain Olam ha'Ba, and the Shulchan Aruch therefore attempted to cover each opinion (that is, he was "Choshesh" for the different opinions). Therefore, Milah is required for the baby that died because this will help the baby enter the next world according to Rav Nachman bar Yitzchak, and, in addition, teaching one's child to answer Amen will achieve this according to Rebbi Meir.

This answer is not necessarily consistent with Rav Moshe Feinstein's words, but it does explain the Shulchan Aruch.

Kol Tuv,

Dovid Bloom