Yotzei Dofen

The gemara on nidda 40A starts off asking what the reason for the rabbanon is that they hold that the woman is not temaia leida after delivering via C-section (yotzei dofen). The gemara answers that they learn it out from "isha ki sazria v'yalda." R' Shimon though needs the pasuk to teach you that a woman who delivers just a shilya is going to be temaia (Rashi explains why the Rabbonon don't need a posuk for that). The gemara then turns around and asks on R' Shimon how he knows that the woman is temaia leida.

The Ritva asks the obvious question - had the Torah not written anything what would we have thought? First the gemara says that the Chachamim need a posuk to tell you that it's not a leida implying that in the absence of a posuk we would have assumed she was temeiah. Then the gemara says that R' Shimon needs a posuk to tell you that she is temeia. Only one of them should need a posuk! The Ritva suggests an answer that really yotzei dofen should not be temaia but R' Shimon has a pasuk which seems to prove that she is so the Gemara starts off questioning the Rabbonan about why she's not if R' Shimon has a pasuk that says she is temaia. He says that this is "derech hatalmud b'mkomos harbe." That may be true but it still doesn't seem to flow properly.

Tosafos Harosh seems to be answering the same question. He doesn't say much. He comments in d"h R' Shimon may taamei that (in my best translation) "That is to say why doesn't he darshen the passuk like the Rabbonon." It sounds like he's saying just the opposite of the Ritva that absent a passuk R' Shimon would be right but the Rabbonon have a pasuk that says that she should be temaia so now the gemara asks how R' Shimon knows it. This does seem to be pashut pshat but the problem is that it takes us back to our original question. I think that what he means is that the gemara explained that R' Shimon needs the Rabbonon's passuk to teach that the woman is temaya if she delivers just a shilya. The gemara is now asking, "How did R' Shimon know to darshan the passuk that way and not to darshen it to exclude a yotzei dofen. Why didn't he just learn like the Rabbonon and make the lady tehora if she delivers just a shilya." Then the gemara says that he couldn't learn like that because he has another passuk.