The correct translation in Isaiah 44:6 for those various names of God is:
This is what Yahweh says:
I am the first and the last,
and there is no Elohim except me.
All of these are names of God, not a Davidic king of Israel.
OT naming conventions were very important and very meaningful. Remember that Israel, the people, rejected Yahweh being their melek. They wanted human kings like all the other countries. Think of the foreshadowning to when Jesus came in the flesh and the remnant, Judah, rejected their Savior. That Isaiah verse is pregnant with meaning.