What did Yehoshua read?
Radak, Malbim: He read all the Mitzvos, Mitzvos Aseh and Lo Sa'aseh.
Metzudas David: It was the Brachos and curses - v'Hayah Im Shamo'a" and "v'Hayah Im Lo Sishma."
Why does it say "veha'Ger ha'Holech b'Kirbam"?
Radak: Nochrim were constantly converting during the journeys in the Midbar, due to the wonders that they heard and saw.