1. Is the angel of the Lord in Joshua 5 Jesus?
Yes, I think this is Jesus.
Reason 1: In the New Testament, when a man bows before an angel, the angel stops him and says to worship God. This angel doesn't stop Joshua.
Reason 2: the ground is holy because God is there, as He was at the burning bush, when Moses takes off his sandals.
Counterargument: There are several examples in the OT of godly men falling down before other men in submission, with no worship implied. Even the word for "worshiped" in 5:14 is used of David bowing to Jonathan and Mephibosheth.
So it isn't conclusive, but Jesus IS the commander of the Lord's army (Rev 19:11).
Either way, the main point of the text is to see Joshua's submission to God. He is not an autonomous national leader conquering in his own will or power.
ISSUES OF FAITH
2. Faith: Joshua's faith is revealed by his obedience. Rahab's and her family's obedience is revealed by their waiting in their house for rescue. This is James' point in James 2.
3. There is no middle ground between faith and lack of it. Being a good person doesn't get you partway spared from God's judgment. You decide to either stand in Rahab's house for protection, or find some way to protect yourself in the city (which gets you killed). You decide to either stand under Christ's protection from God's judgment for sin, or find some other protection.
ISSUES OF LIVING
4. Jericho is devoted to utter destruction. No plunder or spoil, except for God's treasury. This is the firstfruits principle: the first of what God gives to us, we return to Him.
General observations on Joshua
1. Like the issue of slavery in the Bible, it is easy to be embarrassed or ashamed of what Joshua actually did: killing all men, women and children in every city. We need to remember that all of us are born in sin, which deserves death. God was judging them, as He did Sodom and Egypt. God wanted this, not Joshua, and it was just.
2. Joshua is a transition book between the Law and the prophets, as Acts is a transition book between the Gospels and the Epistles. Each one deals with Israel's/Church's mission to "take the land" (disciple the nations). Look to Acts for examples as you talk through the lessons (Ananias and Sapphira are like Achan, for instance.) The conquest (and the church's discipleship) is rapid and seems total (Josh 11:23; Col 1:6), but it is incomplete and there is much yet to do (Josh 13:1; Acts 28:25-31; Rev 19:15).