1)
Why does it say "va'Ynachem la'Vetach"?
1.
Radak #1: "V'Lo Fachadu" (they did not fear anything in the Midbar). Even though it says "va'Yir'u Me'od va'Yitz'aku" (Shemos 14:10, before Keri'as Yam Suf), that was only for a moment. Immediately, Moshe promised them that all their enemies will die, and so they saw - "v'Es Oyveihem Kisah ha'Yam."
2.
Radak #2: When they passed through the sea on the dry land, they did not fear.
3.
Malbim: This is unlike flock in the Midbar, which fear evil beasts. They did not fear, for "v'Es Oyveihem Kisah ha'Yam."
