But Paul clearly applies the idea to all people. For example:
Rom 6:16-23 : 16 Don't you know that when you offer yourselves to someone to obey him as slaves , you are slaves to the one whom you obey — whether you are slaves to sin, which leads to death, or to obedience, which leads to righteousness? 17 But thanks be to God that, though you used to be slaves to sin, you wholeheartedly obeyed the form of teaching to which you were entrusted. 18 You have been set free from sin and have become slaves to righteousness.
19 I put this in human terms because you are weak in your natural selves. Just as you used to offer the parts of your body in slavery to impurity and to ever-increasing wickedness, so now offer them in slavery to righteousness leading to holiness. 20 When you were slaves to sin, you were free from the control of righteousness. 21 What benefit did you reap at that time from the things you are now ashamed of? Those things result in death! 22 But now that you have been set free from sin and have become slaves to God, the benefit you reap leads to holiness, and the result is eternal life. 23 For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord.
These are some of multiple examples, Paul is not only referring to himself.
A slave does not have free will; only a servant does. Do the rest of the Apostles identify themselves as slaves? No; as servants - servants with a free will who can leave the employ of their Master at will.
As you can see from above, Paul at least partially disagrees. This is where all the technicalities of what "freedom" is come into play. "Freedom" is one of those words that can be defined in apparently contradictory ways, and yet I would agree with both in context. Therefore, freedom is always limited, but to very different degrees, and at different times. So, it always depends on the context.
I could only find one other relevant reference to slavery by another Apostle:
2 Peter 2:19 : They promise them freedom, while they themselves are slaves of depravity — for a man is a slave to whatever has mastered him.
In both directions, this would appear to completely agree with the ideas Paul espouses. If Peter thought Christ was his master, and I'm sure we both think he did, then he doesn't appear to think that he was free to leave Him.
The example of the prodigal son has no meaning under Reformed theology except as a mechanical exercise - ...
Every story in the Bible has meaning to us. In many cases it is just a different meaning than to you. You are apparently saying that a parable has more meaning if it can be interpreted as giving man more powers in self-determination. That is not my view of "meaning". :)
If man is made in God’s image, then why would he throw the bulk of humanity away? I understand about the clay, but men are not pots, they are the pinnacle of His creation.
Then apparently you do NOT understand about the clay. :) I mean, it was God Himself who made the analogy. Why would He say that if not to give us a clear picture of the level of His sovereignty? God is free to do with His creation ANYTHING He chooses.
In order to understand Romans 6, it is necessary to go back to the first chapter and see how Paul goes.
Rom 1:1 Paul, a slave of Christ Jesus, called to be an apostle and set apart for the gospel of God
Slave of Christ Jesus: Paul applies the term slave to himself in order to express his undivided allegiance to the Lord of the church, the Master of all, including slaves and masters. “No one can serve (i.e., be a slave to) two masters,” said Jesus (Matthew 6:24). It is this aspect of the slave-master relationship rather than its degrading implications that Paul emphasizes when he discusses Christian commitment.
Rom 6:17 But thanks be to God that, although you were once slaves of sin, you have become obedient from the heart to the pattern of teaching to which you were entrusted.
In contrast to humanity, which was handed over to self-indulgence (Romans 1:24-32), believers are now willingly entrusted to God’s pattern of teaching, that is, the new life God aims to develop in Christians through the productivity of the holy Spirit. Throughout this passage Paul uses the slave-master model in order to emphasize the fact that one cannot give allegiance to both God and sin.
Addiction to vice is akin to slavery to the body; to make the choice to cleave to God is obviously not true slavery. Paul is merely using a literary device to make his point.