1)

(a) The Rambam (Hilchos Ishus 1:3) writes that once a woman becomes Mekudeshes -- even though she has not yet entered her husband's home -- she is an "Eshis Ish" and any other man who has relations with her other is liable for the capital penalty.

I assume this is what you mean, Josh, by Kidushin only but not full marriage.

The Rambam does not mention that if a child was born from these forbidden relations that the child is a Mamzer. I do not know why, but still it must be that the child is a Mamzer because any child born from relations of an Eshis Ish with a man other than her husband is clearly a Mamzer.

The Gemara (Yevamos, end of 69b) discusses an Arusah who became pregnant. (An Arusah is a betrothed woman who has done Kidushin already but has not yet done Chupah (i.e. Nisu'in).) Rav says that the child is a Mamzer. Rashi writes that the baby is certainly a Mamzer, because the majority of people in the world are not this woman's "Arus" (let us say "fiance," but this term means the husband during the state of betrothal, after Kidushun but before Nisu'in), so we assume that the child was conceived from a man other than the Arus. The Shulchan Aruch (Even ha'Ezer 4:27) writes that if she says that the baby was conceived from the Arus but the Arus contradicts her and says that the baby was not conceived from him, the Halachah is that we believe the Arus and the baby is a Mamzer.

This is a proof that if a woman has Kidushin only but not full marriage and then she has a child by another man, the child is a Mamzer.

(b) I found an explicit source in Chazal (although I think the Din is actually obvious and does not need a proof) that the child born from another man -- after Kidushin and before full marriage -- is in fact a Mamzer. This is from Maseches Kalah (printed in the standard Shas in the same volume as Maseches Avodah Zarah) which relates the account of a woman who confessed to Rebbi Akiva that when she had Chupah with her husband she became a Nidah and consequently her husband separated himself from her. Then she had relations with the groomsman and became pregnant. Rebbi Akiva said that the child is both a Mamzer and the child of a Nidah.

(c) Concerning the child born from a Yevamah l'Shuk and a man other than the Yavam, it should be pointed out that there are opinions in the Gemara that this child is a Mamzer. See Yevamos 92b where Rav states that if somebody other than the Yavam marries the Yevamah, the Kidushin does not take effect at all and they do not require a Get to dissolve the union. The rule is that whenever Kidushin is not "Tofsin" and no Get is required (for instance, if sombeody tried to marry his sister, etc.), any child born from that union is a Mamzer. According to this, the child of a Yevamah l'Shuk is a Mamzer. Abaye in Yevamos 49b was in doubt about whether the Halachah follows Rav.

In addition, the Gemara in Nedarim 75a states that a man who has relations with the Yevamah l'Shuk is not liable for Sekilah. It is not the same as the one who had relations with the Na'arah ha'Me'urasah. So Zikah is not quite equivalent to Kidushin.

(d) You are right, Josh, that the reason why the child of a Yevamah l'Shuk from another man is not a Mamzer is because it is only an Isur Lav.

However, I found, bs'd, that Zikah may actually be stronger than Kidushin. See the Ran to Nedarim 74a (DH v'Rebbi Yehoshua) who writes that Zikah is stronger than Erusin (the same as Kidushin). Afterwards, the Ran writes that Zikah might not really be like "Kenusah"; she has not actually entered the husband's home. However, the Tosfos Yom Tov there (Nedarim 10:6, DH Rebbi Yehoshua) disagrees with the Ran. He writes that we have never found anywhere that Zikah is only like an Arusah. According to the Tosfos Yom Tov, Zikah means that she is considered as if she has entered the house of the Yavam.

So there seems to be a certain paradox here. Even though the baby after Zikah is not a Mamzer, still Zikah is considered to effect a full marriage, while Kidushin does not.

2) Regarding your second question, I found a different way of understanding the Kidushin of Bi'ah than the way you were thinking of, Josh.

(a) This is based on the Talmud Yerushalmi at the beginning of Perek 14 of Maseches Yevamos. The Mishnah in Yevamos 112b states that if a Cheresh (deaf and dumb) married a regular woman and he wants to divorce her, he may do so on the spot because the Kidushin does not take effect since he lacks Da'as. The Rashba (and see also Nimukei Yosef) there cites the Yerushalmi that this applies only if the Cheresh did Kidushin with Shtar or Kesef but if it was Kidushei Bi'ah, the Kidushin works. The Beis Yosef (Even ha'Ezer 121, DH uM'Sh O she'Naflah) writes that he did not see any Poskim who wrote like the Yerushalmi.

(b) At any rate, we see from the Yerushalmi that Kidushei Bi'ah is actually stronger than the other ways. I found that the Chidushei Basra (Altuski) to Yevamos (page 229, DH v'Lich'orah) gives a simple explanation of the Yerushalmi. The Cheresh is a person with a very limited understanding, especially of abstract concepts. He cannot grasp the idea that one may marry a woman through giving her money or a piece of paper with words written on it. Therefore, he does not possess the Da'as required to make Kidushei Kesef or Kidushei Shtar work. However, he can understand the idea that marriage can come about through Bi'ah because he understands that an action of Ishus creates a new situation of Ishus between husband and wife, and that they are thereby married.

(c) We can understand from this that Bi'ah is appropriate to create Kidushin, because it is a Ma'aseh Ishus.

(d) Here are further sources which suggest that Bi'ah is a Kinyan for Kidushin, and it actually possesses strengths that Kesef and Shtar do not possess:

The Gemara in Kidushin 10a asks: does Bi'ah make Erusin or Nisu'in? We see that the Gemara entertains the possibility that one act is sufficient to effect the entire marriage. The Gemara never entertained this possibility concerning Kesef or Shtar. The latter two certainly do no more than Erusin, and something extra is certainly required in order to make Nisu'in.

(e) The Gemara in Kidushin 2b states that the reason why the Mishnah (2a) uses the word "Derachim" and not "Devarim" is because the word "Derech" is applicable to Bi'ah. The Gemara says that even though "Derachim" is not applicable to Kesef and Shtar, still we say "Derachim" because it is applicable to Bi'ah. The Gemara asks: how can it be appropriate to use a word which is only relevant to the minority Kinyan of Bi'ah and not to the majority Kinyanim of Kesef and Shtar? The Gemara answers that Kesef and Shtar are "Tzorech Bi'ah," they are necessary in order that Bi'ah will be permitted. The purpose of Kidushin is to allow Bi'ah, so it is logical to use a word in the Mishnah that applies only to Bi'ah.

(f) Rashi to Kidushin 19a (DH d'mid'Oraisa) writes that a minor from age 9 and up may effect Yibum or Kidushin mid'Oraisa with Bi'ah. This is also stated by the Avnei Miluim 43:2 (DH ub'Zeh) in the name of the Shitah Mekubetzes (Kesubos 73b) in the name of Rashi, first edition, that the Bi'ah of a minor over the age of 9 can effect Kidushin. This all shows that since Bi'ah is an appropriate Kinyan mid'Oraisa for Kidushin, it can work where Kesef and Shtar cannot work.

(g) It seems to me that the above opinion of Rashi fits well with the Talmud Yerushalmi that we saw earlier, that a Cheresh also can do Kidushin d'Oraisa through Bi'ah.

Kol Tuv,

Dovid Bloom