Isaiah 59:20 essentially is saying that the Deliverer (Christ) will come and deliver those in Jacob who turn from pesha ("transgression" or "rebellion"). When Paul points to this verse in Romans 11:26, he utilizes the Septuagint translation which has a slightly different take on this prophecy. Two things are different: the Deliver will come and 1) banish 2) ungodliness from Jacob.
Which is it? Does Christ come and deliver those in Jacob who turn from transgression/ungodliness, or does Christ Himself do the work of banishing transgression/ungodliness from Jacob? The answer is: both. Christ will deliver those who turn, but it is also His own work of banishing that causes them to turn. Paul's flexible quotation is highlighting both aspects of that deliverance.
But what do they turn from? Transgression or ungodliness? The Hebrew word pesha here means rebellion, violating a covenant boundary in a presumptuous and high handed way. Both the Septuagint translation and Paul pick up on the sense of the word here by using the Greek word "asebeia", translated "ungodliness", which means irreverence against God. So what Isaiah is saying is that rebellion/irreverence against God will be banished from Jacob by Christ, and that those turn from rebellion/irreverence will be delivered.
Having said all that, I understand the rebellion/irreverence against God not to be the mere act of sinning against God, but rebellion against God's word and truth. God's word declares that the only kind of righteousness He accepts is the perfection of love, and that therefore all humankind are sinful, guilty and worthy of death. God's word further declares that in His Son Jesus Christ an atonement has been made and the everlasting righteousness that He requires has been provided, and that we must trust ourselves to this finished work of Christ alone in order to be saved. Those who refuse to believe that they are damnable sinners and will not trust in Christ are the rebellious/irreverent ones, and those who tremble at God's word, accept His verdict on their lives and trust in Christ alone for deliverance are those who are not rebellious/irreverent against God. Yes, everyone sins, and so in a sense all are rebellious/irreverent, but this verse is referring not to the general sins of mankind, but to the rebellion/irreverence that refuses to accept guilt and humbly trust in Christ. Think of all the passages in the literature of John that talk about this specific rebellion/irreverence (John 1:11-13, 3:19-21, 5:24-47, 7:7, 17-18, 8:37-47, 12:35-50, 17:6-16, 18:37, 1 John 1:5-10, 2:22-23, 4:5-6, 5:9-12, 19-21, 2 John 1:7-11, etc.).
Thus I take Isaiah 59:20 to be referring to the banishing of unbelief in the gospel from Jacob, and, that those who turn to Christ from their rebellious unbelief will be delivered. Both aspects--the banishing and delivering--is the work of Christ.
I'm asking us to think more deeply about rebellion than the mere act of sinning in general, but rather that deeper rebellion against God's truth that refuses to admit sin and believe in Christ.
The connection with metanoia I trust is obvious.
I hope this helps you, C----,