Parshas Shemos:Can A Woman Be A Mohel

My shiur last week discussed whether a woman can do mila, In Parshas Shemos we read how Tzippora gave her son a bris. The gemara in Avodah Zara (27a) has a machlokes whether a woman can be a mohel. Teh gemara asks, according to the opinion that a woman can't do teh mila, how do you understand the ma'aseh with Tzipporah? The gemara gives two answers, either a) Tzipporah asked someone else to do it b) Tzipporah started and Moshe finished the mila.

Machlokes Rishonim

How do we pasken? There is a machlokes Rishonim on this issue. Tosafos brings the opinion that a woman can not do the mila. This is also the shitta of the S'mak and Hagahos Mordechai. The Rif and Rosh (at the end of Perek 19 in Shabbo) and the Rambam pasken a woman could do the mila only if a man is not present. If there is a man around, the man should do the mila.

Mechabeir vs Rema

Both the Mechabeir and Rema (Y.D. Siman 264) pasken a woman could do the mila . However, the Rema adds on that the minhag is a woman should not do the mila. The Shach asks what is the Rema adding on to the Mechabeir. The Mechabeir would not disagree with the Rema.

The Aruch Hashulchan writes that the nafka mina between teh Mechabeir and rema is in acase where there is no male mohel in town but there is a male mohel in a different town. According to the Mechabeir one would not need to go to a different town to find a mohel and according to the Rema one should go to the other town because that is how the minhag evolved. Accordingto this the Rema would hold there is nothing wrong with a woman doing the mila, just teh minhag is that she doesn't.

The Sefer HaBris gives a different hesber based on the Ra'avyah. Teh Ra'avyah understands teh Rema that he is being machmir for the shitta that a woman is posul. Therefore, we don't use a woman because maybe she really can't do the mila. The Mchabeir would hold that m'ikkar hadin a woman is kosher to do the mila but it is better to use a man.

Tichilas B'P'sul V'Sofo B'Kashrus

The gemra in Avodah Zara suggested that Tzipporah started the mila and Moshe finished the mila. This leads to the question, can a posul (an akum) start the mila if a kosher mohel finishes the job. Both teh Shut Beis Ya'akov (ayin Pischei Teshuva) and the Ohr Sameiach (hilchos mila)address this question. The both bring a rayah from the gemara in AZ that you could do this. However, the Ohr Sameiach points out that it would not work on Shabbos. On Shabbos, machshirei mila that can be done before Shabbos are not allowed to be done on Shabbos. (l'moshol if the mila knife breaks on Shabbos you can't bring a new one if it involves carrying). Therefore, when a posul starts the mila it has the staus of machshirei mila since there is no mitzvah for the posul to do the mila. This machshirei mila could have been done before Shabbos (even though it would have been before day 8, since a posul mohel is doing the mila who cares when he starts it). Therefore, a posul can't start the mila on Shabbos.

The Sefer Habris brings a teshuva from the Yidei Moshe that disagrees. He says this question is based on the machlokes in Chullin whether "yesh l'shchitah m'tichila ad sof" or "aino ella l'sof". Since we pasken "yesh l'shchitah m'tichila ad sof", we need a kosher mohel to do the whole mila.
Obviously the Ohr Sameiach disagrees with the comparison. I'll leave this question hanging. How would the Ohr Sameiach answer the kasha from shechita?