Why does the Torah not add the words "Daber el B'nei Yisrael"?
Ramban (13:1): It is not necessary to warn Yisrael about Taharah or the Kohanim about bringing Korbanos on their behalf, since they would be only too pleased to become Tahor and to bring the Korbanos. 1
What is the significance of the words "be'Yom Taharaso"?
Rashi: It comes to preclude being Metaher a Metzora at night. 1
Torah Temimah citing Toras Kohanim: Likewise, one may not declare him Tamei at night...
Why does it say "Toras ha'Metzora"?
Rosh: This is the law of Motzi Shen Ra. Tanchuma (1) - this teaches that there is a Bris, that anyone who speaks Leshon ha'Ra will be stricken with Tzara'as. 1
Refer to 14:57:2:2,3 and the note there.
We do not see Metzora'im nowadays. Perhaps, the person receive a spiritual blemish called Tzara'as, or he will get Tzara'as in another incarnation (heard from R. Meir Eliyahu).
What do we learn from "v'Huva El ha'Kohen"? Others do not bring him. Everyone distances from him! Also, he may not enter the Machaneh, and the next verse says that the Kohen leaves the Machaneh to see him!
Ramban, Moshav Zekenim: He may not delay. On the day he is healed, [if he does not go by himself] he is brought Bal Korcho 1 (against his will).
Bechor Shor: If he is far from the Machaneh, they bring him close to the Machaneh. 2
Rosh (13:9), Da'as Zekenim: We read "v'Huva" like 'v'Hu Va' (he comes by himself).
Tosfos ha'Shalem (12): They bring the Metzora's words to the Kohen, that it appears that the Nega departed.
QUESTIONS ON RASHI
Rashi writes that "b'Yom Taharaso" precludes being Metaher a Metzora at night. We should know this already from "b'Yom Hera'os"!