Why was this Parshah not said immediately after the Mishkan was completed (on the first of Nisan), seeing as was important for the camp of Yisrael to be holy and fit for the Shechinah to dwell in it (Ramban)?
Rashi and Ramban: It was, 1 even though the Torah only inserts it here.
Rashi (citing Gitin, 60a): As were seven other Parshiyos
Then why was it said only here?
Ramban: Since the Torah counted the families of Yisrael and to the house of their fathers and separated the Eirev Rav from their midst, it is appropriate to conclude the Din of Asham Gezeilos, including the Din of Gezel ha'Ger that now came into effect.
Rashbam: Since it deals with sending Teme'im out of the camp, it was said as soon as the camps of Yisrael and of Levi were organized.
Why does the Torah mention the word "Machaneh" three times
Rashi: They represent Machaneh Shechinah (inside the hangings of the Mishkan itself); 1 Machaneh Leviyah (surrounding the Mishkan, where the Levi'im encamped) 2 and Machaneh Yisrael (surrounding Machaneh Leviyah), 3 where the four Degalim encamped.
Rashi (citing Pesachim, 67a): Which all three listed Teme'im were forbidden to enter
What exactly does "ve'Chol Tamei la'Nafesh" mean?