What is the significance of "Am ha'Aretz" that Pharaoh mentioned?
Seforno: He meant that the enlightened among the people would not take any notice of Moshe's request anyway, and it was the majority of workers who would be fooled by it, and would put down tools and stop work. 1
Thereby causing Pharaoh a substantial loss (Rashi).
Why does the Torah mention Viduy specifically here, and not by any of the other Chata'os?
Ramban #1, Moshav Zekenim #1: Because, Shevu'as ha'Eidus, unlike the other Chata'os, is Chayav even be'Meizid.
Ramban #2 (citing the Sifri Zuta), Moshav Zekenim #2: The Mitzvah of Viduy actually applies to all the Chata'os mentioned in the Parshah, and the Torah mentions it here because Shevu'as ha'Eidus and Shevu'as Bituy (in Pasuk 4) are not subject to Kares. 1 All the more so Viduy is needed for Chayavei Kerisus!