Yehoshua addresses them in the plural (Eschem
Radak: (Zachor is not a conjugation, i.e. command form.) It is Makor (like an infinitive).
Moshe commanded them to fight and stay with Bnei Yisrael until the land is conquered (Bamidbar 32:29). Why does it say here (verse 15) until they inherit, i.e. they divide the land among the Shevatim?
Malbim #1: So Moshe commanded in Devarim (3:20). I.e. it is proper that they stay until then.
Malbim #2: Initially, Moshe thought that the war will be miraculous. Then, it suffices to stay until it is conquered. In Devarim, Moshe saw that it will be close to natural; this is why he did not say there 'Lifnei Hashem.' Therefore, he stipulated that they stay until the division.
Why does it say "Mani'ach Lachem v'Nasan Lachem"?