Seeing as the word "Tze'akah" is feminine, why does the Torah write "Kamo'hu" and not 'Kamo'hah'?
Seforno and Targum Yonasan: "Kamohu" refers to (the unwritten) night (which is masculine), and what the Pasuk means is that on such a night 1 no cry ("Nih'yasah" is feminine) 2 was ever heard or would never be heard again. 3
Moshav Zekenim "Kamohu" refers to the (unwritten) destruction (Shod and Shever, which is masculine).
Seforno: When peace reigned - to exclude a night in time of war.
The Rashbam explains both words ("Kamohu" and "Nih'yasah") with reference to Laylah, and this is a case of a word which can be both masculine and feminine (even at one and the same time). See Rashi and Rashbam to Bereishis 32:9.
Perush Yonasan: This also answers the question, that the cries in Egypt following Keri'as Yam-Suf were surely no less than those of Makas Bechoros? Only that took place in the day (in the early morning), and not at night-time.
What is the significance of the "Tze'akah Gedolah" that took place in Egypt that night?
Presumably, it was Midah Keneged Midah for the 'Tze'akah' that Yisrael cried out to Hashem because of the harsh subjugation of the Egyptians. 1
See 3:9.
Why specifically in this Makah, does the Torah emphasize that such had never happened before, and would never happen again?
Gur Aryeh (to 9:14): The final set of Makos - Be'achav - is like one unit. What these Makos have in common, is they were unprecedented in scope, and they teach that "there is none like Hashem in all the earth" (9:14). 1
