Why does the Torah mention that a Neveilah renders Tamei whoever touches it both here and later in Pasuk 39?
Rashi (on Pasuk 6), Rashbam and Ramban: The Pasuk is referring to the Neveilah of a non-Kosher species of animal, whereas the Pasuk later is referring to that of a Kosher species. 1
The current Pasuk does not mention Misah, because even if the animal is Shechted, it will adopt Tum'as Neveilos when it dies, whereas the Pasuk later mentions Misah, to indicate that if it is Shechted first, it will not adopt Tum'as Neveilos (Ramban and Rashbam, on Pasuk 39).
Why is it necessary to mention them separately?
Ramban and Rashbam (on Pasuk 26): Because, whereas the former is Metamei even if it is Shechted, the latter is only Tamei if it dies by means other than Shechitah. 1
See Sifsei Chachamim.
Why does the Torah stress "u'le'Eileh Titama'u"?
Rashi, Ramban, Rashbam and Seforno 1 : The Pasuk refers to the species that are about to be mentioned, to preclude fish, birds and locusts from Tum'as Neveilos.
Ramban #2: Refer to 11:8:4:21.
What are the ramifications of the fact that one becomes Tamei?
Rashi (on Pasuk 29): A Tamei person is forbidden to eat Terumah and Kodshim, and to enter the Beis-ha'Mikdash.
How does the Tum'ah go away in the evening?