Why does the Torah write "ha'Megeifah" with a 'Hey' - suggesting that the plague was predestined?
Seforno: It inserts the 'Hey' with reference to Hashem's statement (14:23, following the episode of the Meraglim) "ve'Chol Mena'atzai Lo Yir'uhah" (Whoever angers Me will not see it!"
How did Pinchas get away with his act of zealousness without any of Zimri's friends stopping him?
Rashi (citing Sanhedrin 82b): Many miracles occurred for Pinchas. (Two of them are that they should have screamed for Zimri's tribe to come to his aid, but he did not, and an angel came and struck people of Shimon, so they did not kill Pinchas. There were four others
Targum Yonason: The metal spearhead lengthened to hold both of them and they did not slip off. There were 12 miracles. [Those not brought in the Gemara are] a. He carried them around the camp held aloft on his spear, and they did not separate. b. He carried them around the entire camp and did not become tired. c. The wood of his spear became tough and did not break from the weight. d. An angel switched them round so that Zimri was on top and Kozbi underneath, and everybody saw their shame (the way that they sinned). e. They remained alive until he took them round the camp and put them down, so that he should not become Tamei be'Ohel ha'Meis - and he was a Kohen. f. Their blood congealed, so that it should not fall on him and render him Metamei Meis. See Sifsei Chachamim and Na'ar Yonasan.
How was Pinchas, a Kohen, allowed to kill Zimri and Kozbi and become Tamei?
Rashi (in Pinchas, 25:12), Rosh (11) #1 and Da'as Zekenim (13) #1: He became a Kohen only after he killed them. 1
Rosh (11) #2, Da'as Zekenim (13: They were quivering and Gosesim (close to death), but did not die until Pinchas separated from them. 2 Da'as Zekenim - a Goses is not Metamei.
Rosh: This is difficult, for his father was a Kohen! (Refer to 25:13:2:1). Hadar Zekenim (14) - one of Elisheva's seven Simchos on the eighth day of the Milu'im was that her grandson (Pinchas) was Mashu'ach Milchamah! Da'as Zekenim (13, citing R. Shmuel) - perhaps he was Mashu'ach Milchamah, but he was not a Kohen for Avodah and to pdonkey to his children until now.
Did Pinchas know that they would not die until later?! Perhaps indeed, he knew through Ru'ach ha'Kodesh. Even though Kana'im Pog'in Bo overrides Piku'ach Nefesh [of the sinner], if another Kanai (not a Kohen) could kill them, Pinchas would not be permitted. He did not seek another Kanai. However, Targum Yonason (7) says that he took his spear only after he saw that everyone else was silent. (PF)
The Sifri counts among the miracles that their blood congealed, lest it is Metamei him. They were still alive. Their blood is not Metamei! And according to the opinion (Zevachim 101b) that Pinchas was not yet a Kohen, it is not a problem if he becomes Tamei!
Moshav Zekenim (7): Some answer, if the blood would have still been on him after death, it would be Metamei him. The miracle that he did not become Tamei was so he could become a Kohen [and serve] immediately after this, without need to wait seven days to become Tahor.