“to harden their hearts.” God allows people to harden their own hearts, but because God’s action is the cause of it, by the Semitic idiom of permission God is said to do it (see commentary on Exod. 4:21).
“that they would have no mercy.” The Hebrew text is ambiguous and can mean that the Canaanites would receive no mercy, or that Israel would show them no mercy. However, the pronouns favor the primary meaning being that the Canaanites would receive no mercy.
“as Yahweh commanded Moses.” Here we see Moses giving the command of Yahweh. Yahweh speaks and acts through His agents.