Why does the Torah write "Vehikrivo" (in the singular)?
Rashi: To teach us that only one of the two Korbanos (the Chatas) is crucial for her to eat Kodshim (or to enter the Beis-ha'Mikdash)
Why does it say "v'Taharah "?
Rashi and Rashbam: It implies that, until now, she was Tamei, as far as eating Kodshim and entering the Mikdash is concerned. 1
Even though she was permitted to eat Terumah already the night before (Rashbam).
Why does it say "v'Taharah mi'Mekor Damehah"? Her blood was Tahor before this! And even if it refers to Tamei blood, Tum'as Leidah does not depend on blood. It is even if she gave birth without blood!
Refer to 12:4:5:2.
Da'as Zekenim: It means that she is Tehorah from the law of the source of her blood; normally it is Tamei. Even though now it was Tahor, "v'Taharah" is appropriate, for she was Tamei for eating Kodshim and entering the Mikdash. 1
Ha'Emek Davar: She will be healed; the source of her blood will not spurt any more, even Dam Tahor, which weakens the body; it is a malady.
He did not explain why we need "mi'Mekor Damehah.". In Nidah 18a, it is used to expound five colors of Tamei blood. (PF)