Seeing as in order to be Tahor, the Tzara'as needs to spread over the entire body, what does "Eis Kol Or ha'Nega" mean?
Ramban: The Pasuk means that the Tzara'as covers the area that was initially stricken plus the rest of the body from head to foot - from which we can extrapolate that if the initial area that was stricken turns into a Bohak 1 or heals completely, the Nagu'a is Tamei.
A Tahor plague (See Pasuk 39).
Why does the Torah add "le'Chol Mar'eh Einei ha'Kohen"?
Rashi: To preclude a Kohen with impaired eyesight from examining Nega'im.
Chizkuni (from Toras Kohanim): This excludes covered parts of the body (e.g. the underarm). A Mishnah (2:4) explains how people pose for inspection; we are not concerned for what cannot be seen in this position.
When Tzara'as spreads over the entire body, why is he Tahor?
Moshav Zekenim: When the Nega spreads, it does not enter the flesh deeply. When it does not spread over the entire body, it consumes and descends. 1
Moshav Zekenim: One whose entire body is covered with Tzara'as, everyone knows this, and he need not announce; he is Tahor. (People do not see under his clothes, only exposed areas. Also one whose face and hands are covered with Tzara'as, everyone knows this! - PF)
Nothing can be totally Tamei. (Heard from R. Aharon Lopiansky.)
Seemingly, also if it spreads over most of the body, it should not enter deeply! (PF)
A Mishnah (2:4) explains how women pose to enable inspection of underarms, Beis ha'Ervah and under the breast. How may a Kohen see these?
Perhaps a woman expert in Nega'im sees, and tells the Kohen what to say. The Rambam (Hilchos Tum'as Tzara'as 9:2) says that if a Kohen does not know, a Chacham sees and tells the Kohen to say 'Tahor' or 'Tamei'. (PF)
Tiferes Yisrael (2:4:24): Only the Nega is exposed; the rest of her body is covered. 1 We do not say 'since he is engaged in his work, he will not come to Hirhur' (Avodah Zarah 20b), for here he does not do an action; he merely looks.
We can use a boy 2 below nine years; there is no concern lest Hirhurim bring him to Tum'ah. (PF) Griz (Nazir 57b DH Tosfos) allows a minor Kohen may see Nega'im.
Perush ha'Mishnayos says 'she is naked'; perhaps he means that the area being inspected is bare, i.e. we do not cover it with a very fine see-through cloth. Tiferes Yisrael (3:1:4) holds that also the Kohen must see the Nega, like Rashi and Tosfos (Erchin 3a), unlike the Rambam (refer to 13:12:152:1). However, if the Tzara'as spread to her entire body, the Kohen must see everything to be Metaher! (PF) Mordechai (Yevamos 12:58) brings opinions about whether or not men may check Simanei Gadlus in girls.
Even though R. Meir (Nidah 50a) equates Nega'im to monetary laws, and one opinion (in Tosfos 50a DH Kol) holds that women can judge, Minchas Chinuch (169:13) and Ezras Kohanim (on Toras Kohanim, Tosfos ha'Azarah 1:11) say that a Kohenes cannot rule about Nega'im. I did not see anyone disagree. (PF)