Which son did Tziporah now circumcise?
Rashi (to 4:24) and Ramban (to 4:20) 1 : Tziporah circumcised Eliezer.
Targum Yonasan: She circumcised Gershom. 2
How could Tziporah, as a woman, perform the Milah?
Avodah Zarah 27a: We learn from here that a woman is eligible to perform Bris Milah.
How did she know why the angel wanted to kill Moshe?
Rashi (to 4:24): Because, in the guise of a snake, it swallowed Moshe, first from the head down to his thighs, then from his feet up to his thighs.
What is the meaning of "va'Taga l'Raglav"?
Rashi: She cast it (the Orlah) before the feet of Moshe.
Targum Onkelos: 'And she approached before him' (i.e. the baby? the angel?).
Targum Yonasan: She placed it (the Orlah) before the feet of the destructive angel.
The opinions of Rashi and Targum have a source in this Yerushalmi as well. also refer to 4:25:3:1*, 4:25:3:5 and 4:25:3:6.
Korban Ha'Edah (loc. cit.): In her haste, the flint-knife touched the baby's foot as well, cutting him. Chizkuni - "Raglav" is a euphemism for the place of the Milah. Invert the verse; she touched the flint-knife to the place of the Milah, and cut the Orlah.
What did Tziporah mean when she said, "Ki Chasan Damim Atah Li?"
Rashi: She meant that he (the baby) almost caused her Chasan to be killed. 1
Seforno: In defense of Moshe, she was explaining that she circumcised the baby on account of the condition that he had made with her when he married her, that they would circumcise their sons and produce the blood of the Bris.
Targum Onkelos: 'It is on account of the blood of the Bris Milah that my Chasan has been given (back) to me.' 2
Targum Yonasan: 'The son-in law (Moshe) wanted to circumcise but the father-in-law (Yisro) stopped him; 3 and now the blood of the Bris Milah will atone for my Chasan.'
Yerushalmi (Nedarim 3:9): #2: [She was saying to the Mal'ach,] 'Here you are! Your Shelichus has been performed.'
Yerushalmi (Nedarim 3:9) #3: Presumably she declared [to the baby], 'Ki Chasan Damim Atah Li!'
Riva: The baby is called a Chasan due to the Mitzvah (just as we say Chasan Bar Mitzvah, Chasan Torah), and my husband's blood is [about to be] spilled due to him!
Da'as Zekenim, Hadar Zekenim (both to 4:26): She thought that perhaps Moshe was being killed for blemishing his own Milah, i.e. that she herself was not proper to be his wife. She circumcised the baby, lest it is due to not circumcising the baby.
Yerushalmi, Nedarim 3:9 #4 Who is the "Chasan"? Some say that that it was Moshe, and what Tziporah meant was that the blood of her Chasan was sought through him (the baby).
Yerushalmi, Nedarim 3:9 #5: Others say that what she meant was that it was the baby was the Chasan, and it was through the blood of Milah that she retained him.
