What does "Meihem" refer to?
Ramban #1: It refers to all the above-mentioned Tamei species - to Beheimah, Sheretz and Chayah. 1
Ramban #2 (citing Chagigah, 11a): We learn from "Meihem be'Mosam" that the Shi'ur Tum'ah of a dead Sheretz (of the eight species of Tamei Sheratzim) is a k'Adashah. 2
Why does the Torah specify "wooden vessels"?
Rashbam: To preclude stone vessels and vessels made of earth from the realm of Tum'ah.
Why does the Torah insert the word "v'Taher" after "v'Tamei ad ha'Erev"?
Rashi: He immerses, but remains Tamei for Terumah until nightfall, at which point he becomes Tahor.
Ramban, Moshav Zekenim (citing Toras Kohanim): "V'Tamei ad ha'Erev" refers to a prohibition against eating Terumah and Kodshim. "V'Taher" immediately after Tevilah he is Tahor for Chulin and Ma'aser Sheini. 1
Ramban (DH 'v'Hinei Hizkir), Moshav Zekenim: One may eat Chulin when he is Tamei if he wants!