Nazir 9a - Mistake and Chazara by Nezirus

This is somewhat involved so please read it slowly.

The gemara explains that if one says "i will be a nazir from figs", Beis Shamai considers his first statement of "i will be a nazir" to be an acceptance of nezirus, and his qualifying statement of "from figs" is like a charata on the first statement, which doesn't work by nezirus just as it doesn't work by hekdesh.

The Mahartz Chiyus points out that the Ramban (ma'aseh korbanos 15:1) says that just as we find in nedarim that one cannot retract from kiddushin or avoda zara even within toch kidei dibur, the same concpet would apply to hekdesh that one cannot retract even within toch kidei dibur (the source is a gemara in temura, and the question why it is not listed in nedarim is discussed by the radvaz). The rationale for this is either similar to what the Ran explains in nedarim that by things that people take very seriously and don't just blurt out, the second it leaves their lips it is as if the kidei dibur has already expired; or that we have a special din by hekdesh of k'misiraso l'hedyod that empowers the din of hekdesh to say that as soon as it leaves one's lips it is a done deal (Tosafos in Menachos 71b assumes the latter). Aside from the universally accepted concept of not allowing retraction, there is an entirely different concept that hekdesh b'taos is still hekdesh learned from temura (opinion of Beis Shamai in Nazir 31a) and therefore sheiala which works through rendering it a ta'os would also not apply to hekdesh (Baba Basra 120b). Now based on the pasuk that says "kadosh yihyeh" by nazir, we are able to technically learn both of these concepts from hekdesh.

Now, in our gemara we can view his statement of "from figs" to be either changing his mind which would not render the original statement a mistake, just a change of heart, AND we can view his statement as being a charata rendering his original statemet of "i should be a nazir", a mistake. It is for this reason that the gemara doesn't mention the issue of not allowing retraction by hekdesh/nazir, since that is not sufficient since we can still argue that it should be rendered a mistake. The gemara has to say that Beis Shamai goes lishitaso that rendering it a mistake would also not work.

This all makes sense if we understand the source of not allowing retraction to be like the Ran explains in nedarim, that as soon as hekdesh/nezirus is made, it is the equivalent of the kidei dibur expiring. This sevara would still not preclude the possibility of rendering the statement a mistake and that is why we can still have a machlokes in the mishna Nazir 30b whether rendering hekdesh a mistake would remove the hekdesh. BUT if we understand that the rationale for not allowing retraction is because 'amira l'govoah k'misiraso l'hedyot', meaning we empower hekdesh as if the transaction has been completed (as Tosafos writes in Menachos), this should also preclude the possibility of rendering it a mistake. How then can Beis Hillel hold that since hekdesh can be rendered a mistake and uprooted, sheila would work by hekdesh? Why would 'kimisiraso l'hedyot' only remove the possibility of retraction but would not ruin the possibility of making it a ta'os?