Surrogate Pregnancy (Shvatim as Jews, part 2)

Another explanation to Shimon's "Canaani" wife (Shimon had a son Shaul ben haCanaanis) is that she was Dina. See Rashi, 46:10 and Breishis Rabbah 11. After she was raped by Sh'chem, she refused to leave until Shimon told her that he would marry her. Only then did she leave Sh'chem with her brothers Shimon and Levi. How could Shimon have married Dinah, if they were both children of Leah, and we saw the Rambam lists incest of siblings of the same mother among the laws that bnei Noach must keep? Torah Shleima 46:10 os 68 brings several answers:
1) the only relationship forbidden to bnei Noach was adultery with a married woman, but not incest. This opinion, cited in the name of the Moshav Zekaynim, clearly argues on the Rambam.
2) According to Rabi Akiva, bnei Noach are not forbidden to their sister from the same mother. This is similar to the first answer in substance as well as in the difficulties it presents.
3) Ra"b (Rav Ovadia Bartenura?) "I have not found an answer but I heard that"... the girsa wasn't that Shimon will marry Dinah, it was that Shimon promised her he will marry her off to someone else, possibly one of his sons. (While an aunt is forbidden in Torah law, it isn't prohibited to a ben Noach, and in fact, Moshe Rabbeinu's father Amram was married to his aunt Yocheved.)
4) The main gestation of Dinah was in Rachel's womb. Therefore they were not really siblings from the same mother: Shimon gestated and was delivered from Leah's womb, but Dina was gestated in Rachel's womb and was only delivered from Leah's womb, and thus they do not have the same mother.

This last answer is fascinating. It is mentioned by Tur ha'aruch to 46:10 and 38:2, Targum Yonasan to 30:21, and Daas Zekanim ibid. The even earlier source is the gemara Brachos 60a that Leah didn't want Rachel to have less sons than Bilha and Zilpah, so she davened and Hashem switched the fetuses between Rachel and Leah.

This opinion has a huge impact on the following question: if a woman cannot conceive and uses a donated egg which is implanted, (or any situation of surragacy), is the biological mother - the woman who provided the egg - the halachik mother? There are three main possibilities: that the biological mother (the one who provides the egg) is the mother; the surrogate in whom the baby gestated is the mother even if she is not the biological mother as far as the egg is concerned; and the mother who delivered the baby even if the baby did not gestate [mostly] in her womb.

According to these opinions (that Shimon & Dina were not siblings from the same mother), the woman in whose womb the baby gestated will be the halachik mother. However, many poskim do not pasken this way. For example, Rav Shlomo Goren paskened that the biological mother (of the egg) is the mother. Rav Moshe Feinstein paskened that the mother who delivered the baby is the mother. In Dina's case, her biological mother and the mother in whose womb she developed was the same woman - Rachel. If doctors would take the egg of one woman, implant it to woman 2, then transplant it to a third woman late-term, to Rav Goren the first woman would be the mother, Rav Moshe would probably pasken the third woman was the mother, and to the Daas Zekanim et. al., probably the second woman will be the mother. (Really, maybe the reason for the Daas Zekanim is because the host mother is the same as the biological mother, but from his words, the main issue is that the host mother is the mother, not the biological mother of the egg.)