Gemara

Nazir 41a (Beraisa #1) Suggestion: Perhaps "A razor will not pass over Rosho (of a Nazir)" applies even if he is a Metzora!

Rejection: "(A Metzora shaves) Rosho" - this teaches that he shaves his head even if he is a Nazir.

58a - Suggestion: This Tana permits Hakafas Kol ha'Rosh. "Rosho" teaches that the Mitzvah for a Metzora to shave is Docheh a Lav and an Aseh (of Nazir - "razor Lo Ya'avor Al Rosho" and "Kadosh Yihyeh Gadel Pera.")

Beraisa #2 - Suggestion: Perhaps "Do not round the corners of your head" applies even to a Metzora (who must shave to become Tahor)!

Rejection: "Rosho" teaches that a Metzora shaves even his Pe'os.

Question: What is this Tana's source that an Aseh is Docheh an Aseh and a Lav?

Answer (Beraisa) Suggestion: Perhaps "(Kohanim) will not shave their beards" applies even to a Kohen Metzora!

Rejection: "(A Metzora shaves) Zekano" - even if he is a Kohen (even though there is also an Aseh, "Kedoshim Yihyu lEi'lokeihem")!

Question: Why must the Torah write both "Rosho" and "Zekano"? From "Zekano" alone we learn that the Aseh is Docheh a Lav (regarding a non-Kohen), as well as a Lav and an Aseh (regarding a Kohen)!

Answer: We cannot learn that a Kohen may shave his beard from a Nazir, since Isurei Nazir are weak, because Yeshnam bi'Sh'eilah (a Nazir can annul his vow).

We cannot learn that a Nazir can shave his head from a Kohen, since Isurei Kehunah are weak. They are not Shavah b'Chol.

We cannot learn from them to other Mitzvos (that an Aseh is Docheh a Lav and an Aseh), for the reasons given.

Yevamos 5a: We hold that an Aseh is not Docheh a Lav and an Aseh. We cannot learn from Isurei Nazir that it does, since Yeshnam bi'Sh'eilah.

Rishonim

Rambam (Hilchos Nezirus 7:15): If a Nazir became a Metzora and was cured of the Tzara'as, he shaves all his hair. Since it is impossible to fulfill the Aseh to shave and the Lav for a Nazir not to cut his hair, the Aseh is Docheh the Lav. A Nazir also has an Aseh to grow his hair, "Kadosh Yihyeh Gadel Pera". Everywhere, an Aseh is not Docheh a Lav and an Aseh. Here it is Docheh, for the days of Metzora Muchlat do not count towards Nezirus, so the Aseh "Kadosh Yihyeh" does not apply.

Rebuttal (Ra'avad): The Gemara in Yevamos (5a) says that Isurei Nazir are more lenient because Yeshnam bi'Sh'eilah.

Defense #1 (Kesef Mishneh): There are two reasons why it is Docheh. The Gemara gave one of them.

Objection #1 (against Kesef Mishneh - Minchas Chinuch, ibid.): The Gemara said that we would learn from Nazir that an Aseh is Docheh a Lav and an Aseh, if not for Yeshnam bi'Sh'eilah (Nezirus can be permitted).

Objection #2 (Einayim l'Mishpat Yevamos 5a DH she'Chen): In many places the Rambam says that an Aseh is not Docheh a Lav and an Aseh. He never says that it is Docheh. Here, he says 'everywhere it is not Docheh'!

Defense #2 (Radvaz): The Rambam sought another answer that applies even to Nezirus Shimshon, which cannot be annulled. The Gemara (Nazir 41a) says that Tiglachas Metzora is Docheh the Lav, without addressing the Aseh of Nazir. We follow it, for it is b'Duchtei (in the Masechta centered around the subject, i.e. Nazir). The Ra'avad must hold that since some kinds of Nezirus are Yeshnam bi'Sh'eilah, all Isurei Nezirus are more lenient, even those of a Nezirus that cannot be annulled.

Note: Elsewhere in Nazir (58a), the Gemara says that Tiglachas Metzora is Docheh the Aseh and Lav of Nazir!

Ha'Makneh (on Tosfos Kidushin 34a DH Ma'akah): R. Yosef (in Tosfos) says that a Lav with an Aseh is more severe, and an Aseh is not Docheh the Lav even when the Aseh does not apply. The Rambam disagrees, for he says that Tiglachas Metzora is Docheh cutting a Nazir's hair because the Aseh of Nazir does not apply then.

Defense #3 (Or Gadol Nazir 6:5): The Rambam agrees with R. Yosef. A Lav with an Aseh is more severe even when the Aseh does not apply. The Gemara in Yevamos teaches that in such a case, i.e. when for Reuven there is only a Lav but for others there is also an Aseh, an Aseh is Docheh the Lav if Yeshnam bi'Sh'eilah.

Defense #4 (Teshuvah Avnei Milu'im 22 DH v'Nir'eh): The Rambam holds like the Maharit, who says that Kabalas Nezirus is Mekadesh the person, similar to a Kohen. The Torah put the Isurim on Nezirim, therefore they are not Shavah b'Chol. They are Einam bi'Sh'eilah (one cannot annul the Isurim, only his Neder to be a Nazir). This is like Kodshim, which are considered Einam bi'Sh'eilah until one annuls the Hekdesh.

Question: There is also an Aseh "k'Chol ha'Yotzei mi'Piv Ya'aseh"!

Answer #1 (Avnei Milu'im, ibid.): Since the Torah put Isurim on Nezirim, "k'Chol ha'Yotzei mi'Piv Ya'aseh" does not apply.

Answer #2 (Minchas Chinuch 373 DH v'Ra'isi): If there is a Heter to shave, he does not transgress his vow.

Rambam (Hilchos Tzara'as 8:3): We shave around a Nesek and leave two hairs (all around) so it will be clear if it spread. This is even if the Metzora is a Nazir.

Question: The Ra'avad there said that Isurei Nazir are more lenient because Yeshnam bi'Sh'eilah. This applies also here, so why do we need a verse to permit even a Nazir?

Answer #1 (Mishneh l'Melech): "V'Hisgalach" permits shaving around a Nesek. Perhaps it is not an Aseh (so we need a source to are Docheh Nezirus). Those who count the Mitzvos do not list it.

Answer #2 (Aruch l'Ner Yevamos 5a DH v'Hinei): "V'Hisgalach" is not a real Drashah. A tradition from Sinai teaches that it is Docheh Nezirus.

Note: Perhaps the Drashah is needed for Nezirus she'Einah bi'Sh'eilah, e.g. Al Da'as Rabim or Nezirus Shimshon. The Yerushalmi (6:5) calls shaving around a Nesek 'Tiglachas Mitzvah'.

Question: Nedarim can forbid things needed for Mitzvos. Tosfos and the Rashba say that the Mitzvah is not Docheh the Lav ("Lo Yachel Devaro") because there is also an Aseh "K'Chol ha'Yotzei mi'Piv Ya'aseh". But an Aseh is Docheh a Lav and Aseh that are Yeshnam bi'Sh'eilah!

Answer #1 (Teshuvas Rashba 3:343): The Sugya is Yevamos is not l'Halachah. It merely gives reasons why we cannot learn to other places. Really, Tiglachas Metzora is Docheh the Lav and Aseh of Nazir due to Gadol ha'Shalom; the Torah is lenient to speed Taharas Metzora to permit him to his wife.

Answer #2 (Or Gadol, ibid.): According to the Rambam and R. Yosef, an Aseh with a Lav is more severe even when the Aseh does not apply. Only then, i.e. for Reuven there is only a Lav but for others there is also an Aseh, the Gemara says that an Aseh is Docheh the Lav if the Lav is Yeshnam bi'Sh'eilah.