Gemara

(Beraisa #1): Suggestion: Perhaps "a Ta'ar will not pass over Rosho (a Nazir's head)" applies even to a Metzora!

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

Suggestion: This Tana permits Hakafas Kol ha'Rosh. "Rosho" teaches that the Mitzvah for a Metzora to shave is Docheh (overrides) an Aseh ("Kadosh Yihyeh Gadel Pera") and a Lav (for a Nazir to cut his hair).

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 he shaves even his Pe'os.

Question: What is this Tana's source that (here) 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 also an Aseh forbids this, "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, for the reasons given.

Yevamos 5a - Suggestion: Chachamim learn from "Rosho" (that a Metzora shaves his head even if he is a Nazir) that an Aseh is Docheh a Lav.

Rejection: An Aseh is not Docheh a Lav and an Aseh. We do not learn from a Metzora Nazir, because Isurei Nazir are weaker, since Yeshnam bi'Sh'eilah. For the same reason, we cannot learn from a Nazir that an Aseh is Docheh a Lav.

Rishonim

Rambam (Hilchos Tzara'as 11:3): A Kohen must shave a Metzora both times.

Minchas Chinuch (373 DH v'Lav): It seems that since the Torah required a Kohen to shave the Metzora, the Kohen must be an adult. The Sha'ar ha'Melech (Avodah Zarah 12:1 Sof DH Ra'isi b'Michtav) does not require an adult. He says that the Aseh of Tiglachas is Docheh the Lav and Aseh of a Nazir, also for the Kohen, and it must be done during the day.

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.

Question (against Rambam - Tosfos Yom Tov Nazir 6:5): If there is no Aseh regarding a Metzora Nazir, why did the Torah need to write both Rosho and Zekano?! Perhaps the Rambam holds like the other Beraisa, which says that Rosho forbids Hakafas Kol ha'Rosh.

Minchas Chinuch (373 DH v'Hinei): Even though one is Yotzei Gilu'ach even if he leaves a hair over, and a Nazir who shaves even one hair transgressed, the Mitzvah is to shave every hair. He need not leave over a hair.

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: In Hilchos Nezirus, the Rambam said that because the days of Metzora Muchlat do not count towards Nezirus, the Aseh "Kadosh Yihyeh" does not apply, therefore Tiglachas Metzora is Docheh the Lav. Here we discuss a Musgar, not a Muchlat, so this does not apply!

Answer (Mishneh l'Melech): Toras Kohanim expounds "v'Hisgalach" (instead of v'Gilach) to permit even a Nazir.

Question (Avnei Milu'im Teshuvah 22 DH Kosav): The Tosefta (Nazir 4:3) permits a Nazir to shave, not only Tiglachas Taharah, rather, even around a Nesek, so later we will know whether or not it spread. If so, why do we need "Rosho"? We learn a bigger Chidush from "v'Hisgalach"!

Question: Why may others shave around Ploni's Nesek? They are commanded not to shave (other people, and there is a second Lav if Ploni is a Nazir), but the Aseh does not apply to them!

Answer #1 (Afikei Yam 2:34 DH uv'Emes): It is a Mitzvah for Ploni to shave around his own Nesek. Since he is permitted, his Shali'ach is permitted.

Answer #2 (Minchas Chinuch 374 DH v'Hinei Ha): Whenever a Metzora shaves, it is totally permitted, it is not mere Dichuy.

Afikei Yam (DH veha'Nir'eh b'Zeh): Shaving around a Nesek is Hutrah (totally permitted), but Tiglachas Taharah is Dichuy (only what is essential is permitted). The Mishnah (Nazir 44a) says that shaving is Hutrah totally permitted for Tiglachas Mitzvah. This refers to shaving a Nesek.