What exactly is the law after the first seven days quarantine?
Rashi: If the Nega spread, even if it did not get stronger, or vice-versa, or even it remained unchanged, it is Tamei Muchlat. Whereas if it did not spread but became weaker, it must be locked up again for another seven days. 1
As R. Yehudah learns from "Vehisgiro Shiv'as Yamim Sheinis" (Pasuk 54); whereas according to the Rabbanan, it too is Tamei Muchlat (See Malbim). In any event, it is only if the Nega disappears completely - either after the first seven-day period or the second, that the Beged is Tahor (See Pasuk 59).
What does "b'Karachto Oh b'Gabachto" mean?
Da'as Zekenim (42), Hadar Zekenim (46): It is whether in a place without hair, or a place with hair.
Why does the Torah mention Karachto and Gabachto here? (Even if these were omitted, we would include old and new garments!)
Rashi: In order to learn a Gezeirah Shavah from Karachto and Gabachto written about Nig'ei Adam, 1 to teach us that just as Nig'ei Adam that spread to the entire body are Tahor, so too Nig'ei Begadim.
See Pasuk 42.
What does "Pecheses Hi" mean?
Da'as Zekenim: It is decreasing, It is not enough to tear out the Nega from the garment, for it is totally ruined (Hadar Zekenim 46).
QUESTIONS ON RASHI
Rashi writes that a Gezeirah Shavah teaches just as Nig'ei Adam that spread to the entire body are Tahor, also Nig'ei Begadim. Regarding man, it says (verse 12) "v'Chisesah ha'Tzara'as Es Kol Or ha'Nega me' Rosho v'Ad Raglav." I.e. there was a Michyah in the Nega, and now Tzara'as covered the entire Nega. There is no comparison to garments!
Riva (42): Here it implies that verse 12 discusses covering the entire man's body.