Why does it say "Mipaneinu"? Was Yehoshua addressing people who passed through Yam Suf?
Radak: He was addressing people who left Egypt 1 (and passed through Yam Suf), and people born in the Midbar.
Malbim: Yehoshua referred to himself.
Bava Basra 121b: The decree to die in the Midbar did not apply to Shevet Levi, men under 20 or over 60 (or women).
Why does it mention Keri'as Yam Suf together with splitting the Yarden?
Malbim (23,24): Both of these were wondrous, for two purposes. (a) The nations should recognize Hashem, and fear punishment from Him. (b) Yisrael should recognize His wonders, and have awe of His grandeur. Hashem 'repeated' the miracle with the Yarden, so (a) also residents of Eretz Kena'an, who did not see Keri'as Yam Suf, should know Hashem's strength, and (b) so you should fear Him forever.