ain mevatlin issur l'chatchila

The gemara (Kid 57b) writes that the bird used for taharas ha'metzora which is sent away is permitted to be eaten:

ת״ר, צפור המשתלחת מותרת באכילה, מאי טעמא ושלח אמר רחמנא, ולא אמרה תורה שלח לתקלה

Why do you need a special pasuk to tell you that the bird is permitted?  Even if it was assur, it would be bateil b'rov?

The Torah Temimah (14:7) writes that the concern here is not for the person eating the bird, but rather for the person sending it away.  This is a doraysa source for the din of ain mevatlin issur l'chatchila.

The problem is that with the exception of the Raavad, most Rishonim hold that ain mevatlin issur l'chatchila is a din derabbanan.  How do they deal with this sugya?

1) One can accept the proof as valid, and qualify the views of the other Rishonim:

It's a little strange that the Torah Teminah would be unaware of a chiddush of the Noda b'Yehudah (Mh"T Y.D. 45) quoted by his father in the Aruch haShulchan Y.D. 99:27 , who writes "divrei taam heim." The Noda B'Yehuda writes that the principle of bitul comes into play only when we cannot distinguish between issur and heter.  If I have two items that I can clearly tell apart, ain mevatlin issur l'chatchila because there is no question of which is heter and which is issur.  When dealing with yaveish b'yaveish, dry items that are distinct units, everyone holds ain mevatlin issue l'chatchila is a din d'orsaysa.  The machlokes Rishonim is only in a case where liquids are being mixed and the end result is an entity where the composite parts can never be separated.  

According to this approach, in the case of birds where are are dealing with distinct units, everyone will hold that ain mevatlin issur is a din d'oraysa and so there is no contradiction to any of the shitos.

2) Or, one can argue that the proof itself is flawed.  

It seems that Rashi did not agree with the T"T's reading of the gemara: 

דלא אמרה התורה שלח לתקלה שתהא למכשול עון וילכדנה אדם ויאכלנה

The concern is for the person who might eat the bird sent away, not the act of sending it away, the act of bitul.

What possible " מכשול " could there be once the bird is bateil b'rov?

R' Shimon Shkop in Shaarei Yosher (3:6) explains that there are two dinim in rov:

1) Rov in the case of a mixture where all the items are jumbled together in the same location, in which case the miyut  gets transformed into the rov and it's like it no longer exists;

2) Rov in a case where multiple items all fall under the same safeik, but they are not mixed together in one spot.  Here, the Torah says you can follow rov, but the miyut remains lurking out there.  

All the birds in the world fall into the safeik of whether they are/are not the bird sent away by the metzora, but those birds are scattered everywhere.  The miyut still exists b'metziyus, just the Torah allows a person to follow the rov -- the Torah allows a person to play the odds.  

Playing the odds is never a guarantee.  Had the metzora bird been assur, the Torah would have not allowed it to be sent away given the chance, however, small, that someone might come to eat it.