Why does the Torah use the expression of "Ki Seitzei mimenu ... "?
Targum Yonasan: Because it is referring to a case of Shogeg. 1
The Ibn Ezra also learns the Pasuk in this way, and the Avi Ezra explains that the Torah does not refer to Resha'im.
What is the Halachic difference between the Ba'al Keri mentioned in this Pasuk and the Zav mentioned in the previous Parshah?
Sifsei Chachamim: A Ba'al Keri is only Metamei via touching, whereas a Zav after his second sighting is also Metamei via carrying and via Mishkav and Moshav.
Why is a Ba'al Keri Tamei (even when it is not through sin)?
Ramban (on Pasuk 11): Because, not knowing whether a baby will be born from it or whether it will go to waste - it is like a body from which the Neshamah has been withdrawn. 1
Seforno (on Pasuk 32): Because an emission of Zera comes through improper intent (not in order to have children), due to the [sin that came through] the snake.
In which case it is similar to Tum'as Meis (See R. Chavel's footnotes).
Why does it say here "Kol Besaro"? Also a Zav must immerse his entire body!
Ha'Emek Davar, from the Ra'avad: This hints that the Shi'ur for a Mikveh is enough for him to stand in (three Amos by one Amah by one Amah). A Zav immerses in a spring, and not in a Mikveh.