Why does it say "Aseh Lecha"?
Radak: [Yehoshua is primary;] others circumcise with him. If he alone circumcised, one knife would suffice! Also, in four days he could not circumcise everyone born in the Midbar 1 !
Rashi (Shabbos 89b) says that every man's wife became pregnant with a male after Matan Torah. It seems that they circumcised those sons promptly (refer to 5:2:5:2). Those sons were virtually all the men [above 20] counted at the end of the 40 years! In the Midbar, people died only in the 60th year, or due to a few incidents, e.g. plagues after Korach and Ma'ase Zimri. However, presumably, many males less than 20 entered the land and needed Bris Milah. Bnei Gad and Bnei Reuven said that they will build cities for their small children (Bamidbar 32:16). Bnei Yosef asked Yehoshua for more land, because they had many youths (Bava Basra 118a) (PF).
What are "Charvos Tzurim"?
Bereishis Rabah (31:8): They are sharp rocks. Malbim
Radak: Tzurim is an adjective (sharp). Alternatively, it is a noun that means sharp (blades).
We find this only regarding a Kli that touches a Mes, but not regarding a Kli that touched a Tamei Mes! (PF) Yefe To'ar explains that he did not use iron, for the wound takes longer to heal, or because iron shortens life and Milah lengthens life
Why does it say "v'Shuv Mul"?
Yevamos 71b: This hints to Peri'ah (pulling back the skin, lest he look uncircumcised). This [Mitzvah] was not given to Avraham Avinu.
Radak: Go repeatedly through the Machaneh, to circumcise and command them to circumcise
Why does it say "Shenis"?
Rashi #1: On the night [before] leaving Egypt, a great congregation circumcised together. This was the second such time. All 40 years in the Midbar, the north wind did not blow (Yevamos 72a), and they did not have a nice day to circumcise.
Rashi #2 1 : "Shenis" refers to Peri'ah, which was not given to Avraham Avinu.
Yalkut Shimoni 15: "Shuv" and "Shenis" teach Peri'ah and strands that are essential. (It does not say which is learned from which.) Radak
QUESTIONS ON RASHI
Rashi writes that they did not circumcise in the Midbar, because the north wind did not blow. This was lest it scatter the Clouds of Glory (Yevamos 72a). Hashem could have caused that it blow without scattering the Clouds, to enable Bris Milah!
Radak #1: Since it will be possible to do the Mitzvah afterwards, Hashem did not want to change His conduct of the world. Most miracles are through causes.
Radak #2: They did not circumcise in the Midbar, because they traveled "Al Pi Hashem", and they did not know when they will travel. 1 If they will travel on the day of the Milah 2 , it would be dangerous. We find that Moshe delayed circumcising his son due to traveling! They brought Pesach in the second year in the Midbar, for they circumcised their sons while at Har Sinai. They ceased circumcising only when they left, and were prone to travel at any time.
Malbim (4): Moshe could have told them when they will be in one place for a while!
Seemingly, there is concern for traveling within three days. Some permit bathing the baby on the third day, even if it is Shabbos (Shabbos 9:3), for there is still great pain. If the concern is only for the same day, why did they not circumcise on Friday, shortly before dark (when there is no time to dismantle the Mishkan and for all the Degalim to start traveling before dark)? Even though the pillar of fire enabled traveling at night (Shemos 13:21), surely they would not start traveling on Shabbos, due to carrying or Mechamer (making animals work!