(a) TOSFOS (DH u'Shmuel and DH Teyuvta; see also DH Rav Asi) explains that
Mesirah la'Ba'al actually accomplishes Nesu'in mid'Oraisa.
The Amora'im who say that Mesirah works only for Yerushah do not mean that
all it accomplishes is that the husband inherits the Nedunya. Rather, they
are merely excluding Achilas Terumah. They are saying that it works for
Yerushah (and for the other laws of marriage) but it does not work to permit
the wife to eat Terumah. Even though Mesirah is a type of Chupah, there is
still a fear of "Simpon," and the Rabanan therefore took away the woman's
rights to eat Terumah until she moves in with him.
(b) RASHI (48b, DH Shmuel) takes the opposite approach. Rashi writes that
Mesirah is really not a Chupah at all. Chupah involves Yichud of some sort
in the house of the husband. Mesirah is not a form of Chupah at all. Even
though the Halachos of Na'arah Me'urasah do not apply after the Mesirah
(that is, if she is Mezanah, she receives Chenek like any married woman, and
not Sekilah like an Arusah), that is not because Mesirah is a form of
Chupah, but it is because there is less of a disgrace to the father after
that point, and therefore the punishment is less severe.
Why, then, may the husband annul her Nedarim after the Mesirah if Mesirah
does not create Nesu'in? The Halachah of Hafaras Nedarim should depend on
marriage!
The answer is that Rashi (ibid.) in fact writes that even after the Mesirah,
the husband may *not* annul her Nedarim. How, though, does Rashi understand
the Beraisa here that says that Mesirah removes her from the Reshus of the
father with regard to Hafaras Nedarim, implying that the husband *may* annul
her Nedarim?
The PNEI YEHOSHUA and others explain that according to Rashi, she is neither
in the Reshus of the father nor in the Reshus of the husband. Mesirah only
removes her from the father's Reshus, but it does not bring her into the
husband's Reshus. Neither of them may annul her Nedarim after Mesirah. (This
is why the verse compares her to Almanah Gerushah, for whom no one may annul
her Nedarim.)
This answers a number of other difficult points in Rashi. REBBI AKIVA EIGER
points out that Rashi earlier (48b, DH l'Olam) implies that even according
to those who say that Mesirah makes them completely married, it is *not*
Koneh her for everything; there are certain matters for which she is not
considered married. Regarding what does Mesirah not serve to be Konah her to
him?
The answer is that Mesirah does not make her married with regard to Hafaras
Nedarim. It makes her married to him with regard to Terumah, because there
is no longer any fear of "Simpon," and with regard to other Halachos that
are mid'Rabanan. (It also makes her like a married woman with regard to
Tum'ah, if he is a Kohen, as Rashi 48b DH Mesirasah la'Kol tells us. The
reason for this is that since she has left the house of her father through
Mesirah, there now is nobody to bury her. She therefore is deemed a "Mes
Mitzvah," and the husband may bury her -- as the Gemara explains in Yevamos
89b with regard to a Mema'enes.)
This might also explain why Rashi says that the opinion that holds that she
may eat Terumah after Mesirah also holds that the Rabanan prohibited an
Arusah from eating Terumah only because she might give it to her brothers
and sisters, but *not* because of "Simpon." TOSFOS asks that the conclusion
of the Gemara (on 58b) is that the reason the Rabanan prohibited her from
eating Terumah is because of "Simpon" and not just because she might feed
the Terumah to her brothers and sisters. Rashi should have explained that
the opinion which allows her to eat Terumah after Mesirah holds that there
is no more problem of "*Simpon*" at that point, since that is already the
beginning of Nesu'in, and the husband checks her out before that point. Why
does Rashi explain that Rav Asi, who holds that she eats Terumah, is arguing
with the conclusion of the Gemara later (58b)?
The answer might be that Rashi is following his opinion that Mesirah only
removes her from her father's house but accomplishes nothing towards
bringing her into the Reshus of her husband. Therefore, it would not be
logical to say that her husband checks her for blemishes and that there is
no fear of "Simpon" from the time of the Mesirah. Tosfos, on the other hand,
is following his view that Mesirah is Chupah mid'Oraisa and therefore the
husband checks her out for blemishes before the Mesirah. (M. Kornfeld)