Until when can a girl do miun?

The mishna/gemara in nidda 52a quotes a three way machlokes about the last time that a girl can do miun. First a definition of miun since the gemara didn't give it: A father can marry off his daughter one time m'doraysa. If he's dead then the Rabbonon gave the mother and brothers that power but they gave the girl an out. She can annul the marriage up to a certain point that we discussed in our gemara. Also, if the father already married her off then she was widowed she's called a yesoma b'chayei aviha and he can no longer marry her off m'doraysa. It's the same as if her father was dead and her mother married her off. Now the machlokes about the last time that she is allowed to do miun.

1. The Tana Kama (Rashi refers to this as shitas R' Meir so I will too) - two searos
2. R' Yehuda according to pashut pshat in the mishna and how some in the gemara understand him- ad sheyirbu hashschor (regardless if she had bia before that or not)
3. R' Yehuda according to R' Abahu - once she has bia after two searos or ad sheyirbu hashachor

The thing that is not clear from the gemara is what happens at these three different stages that prevents her from doing miun. That's what I'll attempt to clarify here although I'm sure that there are other opinions that I'm not mentioning.

1. Rashi says that according to R' Meir when she has two searos the kiddushin d'rabonnon that took place when she was a ketana automatically takes effect without anything else happening when she's a gedola. It sounds from Rashi like this is kiddushin d'oraysa. Indeed, that's the way that the Aruch Laner understands it. He also quotes Tosafos in Yevamos on daf 109a (d"h v'hu) who argues and says that R' Meir believes that m'doraysa a kinyan kiddushin needs to be made once she's a gedola just that the Rabbonon made a takana that she shouldn't be able to do miun anymore.

2. I think that according to this opinion it's clear that the whole thing is only m'drabbonon. Even though she had bia and she's a gedola she can still back out. The Ritva explains that this is because we assume that the bia that she did when she was a gedola was only al daas kiddushin rishonim which wasn't a kiddushin d'oraysa so this isn't either. I think that even m'sheyirbu hashachor it still isn't kiddushin d'oraysa. I don't see any reason that anything should change at that point. I think just that R' Yehuda believes at that point the Rabbonon made a takkana that it's too late to do miun. According to this opinion then the only time it's kiddushin d'oraysa is when the man makes a kinyan kiddushin after she has 2 searos with both of them having the intention of doing a new kiddushin and not just a continuation of the old one.

3. Accoring to this opinion, R' Yehuda holds that once yirbu hashachor then it's too late because the Rabbonon made a takana. What about once she has bia after 2 searos. The Ritva says that R' Yehuda holds that we are choshesh that she meant for the bia to be kiddushin and that would effect a kiddushin d'oraysa. Therefore we can no longer allow her to do miun. It sounds like there isn't any major machlokes between the two ways of understanding R' Yehuda. Both opinions believe that there must be a new kinyan with the intention of effecting a new kiddushin but the only question is if we are choshesh that was their intention.

There's actually a lot more on this topic but I really need to work so I'll have to leave it at that. One interesting topic that I didn't have a chance to really look at is if a girl who doesn't have simanim (we checked and we can see that she doesn't have) can do miun once she's 12 years old. I would have assumed that she definitely could but the Hagaos Maymoniis in hilchos geirushin 11:4 says that's not necessarily true. You can take a look there for more on that issue.