Why is there an extra 'Lamed in the word "le'Chol Cheil Par'oh"?
Rashi: The Torah often adds a 'Lamed'. 1
Rashi: Like we find in Sh'mos 27:3, 19, and in Bamidbar, 4:32.
Ramban: See above, Pasuk 9.
Ramban: In which case, the word "le'Chol" is missing a 'Vav' ('u'le'Chol'); and it is grammatically correct to add a 'Lamed' following the word "Vayechasu es ha'Rechev" - as we find in Yeshayah, 11:9 and in Terumah Sh'mos, 26:14 (See also R. Chavel's footnotes).
What is the Pasuk referring to when it adds "le'Chol Cheil Par'oh"?
Rashi: It refers to the chariots and the riders that are mentioned earlier in the Pasuk.
Ramban: Refer to 14:28:1:2.
Seforno (in 15:9): It refers to the chariots and riders (private citizens) who followed the army to the battle-ground to share in the spoil.
Why does the Torah add the words "Ad Echad"?
Chizkuni: It wants to stress that not even one soldier survived. 1
Da'as Zekeinim and Hadar Zekenim: It means that one man did survive - Par'oh. 2
As the Pasuk writes in Tehilim (106:11) "Not one of them remained" (Chizkuni).
And when the Pasuk writes in Tehilim (106:11) "Echad meihem Lo Nosar", it means 'not one of his nation.'