2. What does Paul mean in 1 Cor. 5:5 when he decides to hand an immoral man over to Satan so that his flesh will be destroyed and his spirit saved? The Bible warns us that living willfully in sin and refusing to repent and turn to God’s ways will eventually destroy our salvation, so it appears that Paul is letting such a man live as he wants. But that will threaten his salvation. So I know I misunderstand something. Please clarify.
1. If you look at the context of 1 Corinthians 3:15 you will see that Paul is talking about evangelism. Specifically, he mentions us laboring as Christians planting and watering and God causing our work to grow. He then switches his analogy to building a building, noting that if we build (the church? our evangelism/conversions?) on a foundation other than Jesus Christ, the foundation is not sound, the quality of the work is not good and the building is likely not to stand the test of "fire". It is at this point that we come to the verse you are trying to interpret. "If anyone’s work is burned up, the builder may survive, but that which he built will be lost. Here is how I see this passage. Paul is talking about the building of the church. Those who evangelize (build the church) for selfish or worldly ends will see their fruit lost. In context, Paul is talking about building on personalities ("I am of Paul" or "I am of Apollos"). If we bring people to Christ using worldly methodologies, then such conversions are likely to be false. Those who are baptized will either remain as false disciples within the church or will fall away from God and leave. Building with poor materials does not necessarily mean that we will lose our salvation ("but he will be saved") but our work will not be fruitful. He is not talking about works salvation. None of us live lives which "measure up" to God’s standards. All of us are sinners and none of us will build perfectly. All of us need the grace of God in order to be saved. Paul is not talking about whether the quality of our work will be sufficient to save us or not. None of us are saved by our works. He is talking about whether those who we save are likely to remain faithful. Bottom line, we need to build the church on Jesus Christ, not on human leaders or worldly standards.
2. In 1 Corinthians 5:5 when Paul advises the church to hand the immoral brother over to Satan, he is commanding that they disfellowship him. In other words, Paul hopes that the discipline of the church–the withdrawal of Christian fellowship–will bring the one who commits blatant public sin to the point of repentance so that he can be saved. He is using what I assume is idiomatic language here. He is not literally telling them to turn the brother over to Satan, but this is descriptive language representing that the disciple will be removed from the church and returned to the world. Like Paul says, "Put away the evil person from among yourselves." The phrase "for the destruction of the flesh" is also not to be taken literally. Paul is hoping that if the sinner is disfellowshipped, he will come to his senses and turn away from his sin, therefore destroying the flesh (slavery to sin) in the sinner. If he is renewed to repentance, as Paul hopes, then "his spirit may be saved in the Day of the Lord." Please do not take Paul’s words in their fully literal sense here. Of course, like you say, if we live willfully in sin and refuse to repent we will be lost (Hebrews 10:26). This is why Paul speaks so strongly and why he advises the man to be disfellowshipped. His situation is so desparate that his only chance to not lose his salvation is to be disciplined by the church. Paul DEFINITELY is not letting the man live as he wants. This is exactly the opposite of what Paul is teaching in 1 Corinthians 5.