That's a lot of words without addressing the actual question.Okay, here is the full quote in context:
19 For I through the law am dead to the law, that I might live unto God.
20 I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me.
21 I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain.
I see quite a lot of figures of speech and metaphorical language there. For example, what does he mean when he says, “I am crucified with Christ”? Obviously it is not meant to be taken literally. I take that to mean that he lives a life of sacrifice to God. Then in the continuation he adds, “nevertheless I live; yet not I, but Christ liveth in me,” which is another obvious metaphor and figure of speech, for which he provides the explanation as follows: “and the life which I now live in the flesh I live by the faith of the Son of God.” In other words, what he means by “Christ liveth in me” is that he lives by the faith of the Son of God. So where do you read into that him saying that he already has eternal life while living in the flesh here in this world? I don’t. Elsewhere Paul has said:
1 Corinthians 9:
27 But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.
So Paul himself recognizes that his salvation is not guaranteed unless he “keep under his body” and “bring it into subjection”. So what he means by “Christ liveth in me” in Galatians 2:20 cannot be that he already has eternal life in this world, otherwise why should he want to be fearful that if he doesn’t do the right thing, he might himself become a “castaway”? So going back to you original question:
It depends on context in which that expression occurs. In the context of Galatians 2:20, it means that he lives by faith in Jesus Christ. That is the explanation that himself provides. He is not saying that he already has eternal life while living in the flesh here in this world. That is the answer to your question.
What literal truth is being expressed by "Christ lives in me"?
If you are saying that means to live by faith in Jesus, you did not support that idea