1)
Why does the Pasuk switch from singular ("Tzaror es ha'Midyanim") to plural ("ve'Hikisem osam")?
1.
Moshav Zekenim and Ba'al ha'Turim (citing Midrash Rabah): It begins in the singular because it was Moshe's personal vengeance - since Midyan (Kozbi's father had commanded her to commit adultery specifically with the greatest among them - Moshe, 1 and switches to plural (Yisrael and and not Moshe himself), since Moshe grew up in Midyan, and it was not fitting for him to fight them personally.
1
Only Zimri took her and told her that he was equal to Moshe - Midrash.
