1)
Why does Yehoshua mention his death here?
1.
Malbim: Even after I die, Hashem's Hashgachah
2)
What is the meaning of "vi'Ydatem b'Chol Levavchem uv'Chol Nafshechem"?
1.
Radak: Pay attention to all the good things that happened to you, and you will know that it is all from Hashem.
