“This is what Yahweh says.” In Isaiah 50, the first three verses (Isa. 50:1-3), and the last two (Isa. 50:10-11), are Yahweh speaking. The middle verses, Isaiah 50:4-9, are the Messiah speaking.
“divorce.” Israel sinned so egregiously that God divorced her (cp. Jer. 3:8).