It already said (23:2) that Yehoshua gathered all Yisrael!
Radak: He gathered them again and rebuked them twice, so they will be careful to guard the Torah.
Why did he gather them to Shechem, and not to the Mishkan in Shilo?
Radak: Perhaps Hashem commanded so, so they make the Bris in Shechem, for (a) Avraham delayed there when he entered the land
Hashem put fear on the surrounding cities and they did not pursue Bnei Yakov [to avenge killing all the men of Shechem] (Bereishis 35:5). (PF)
Before this, Avraham bought Me'aras ha'Machpelah! Perhaps that is not considered the start of the inheritance, for he bought it for the sake of burial (even though Efron forced him to buy also the field
What is the meaning of "va'Yitzyatzvu Lifnei ha'Elokim"?