In Romans 7:22, Paul said that he delighted in obeying the Law of God and in Romans 7:7, Paul said that the Law of God is not sinful. If Romans 7:5 were referring to the law of God, then please explain how it makes sense to you to think that Paul delighted a law that stirred up sinful passions in order to bear fruit unto death and that such a law is not sinful. When Romans 7:7-8 speaks about sin acting upon the Law of God, it is my interpretation that it is referring to the law of sin acting on the law of God because Paul described the law of sin as hindering him from doing the good of obeying the Law of God that he wanted to do, so the law of sin works through the Law of God rather than being the Law of God. In Romans 7:21-25, Paul was making concluding statements about the law of God and the law of sin, which implies that the previous verses that he was concluding from also discussed the Law of God and the law of sin, so perhaps it would help if you were to explain which are verses that you interpret as referring to the law of sin.
The first thing that Paul did after the Jerusalem Council was to have Timothy circumcised (Acts 16:3), so I don't think that it is correct to interpret Acts 15 as being a ruling against circumcision. The Apostles did not have any authority to tell Gentiles not to obey any of God's commands, so yes, I think that they would command physical circumcision for both Jews and Gentiles. For example, a Gentile needed to become circumcised in order to eat of the Passover Lamb (Exodus 12:48), and in Paul encouraged Gentiles to continue to observe Passover (1 Corinthians 5:6-8). In Acts 15:19-22, the expectation was that Gentiles would continue to learn about how to obey Moses by hearing Him taught every Sabbath in the synagogues.
In Matthew 7:23, Jesus said that he would tell those who are workers of lawlessness to depart from him because he never knew them, so knowing Christ is the goal of the law for righteousness for everyone who has faith (Romans 10:4). In Galatians 3:26, it says that for in Christ we are all sons of God through faith, and this needs to be balanced with 1 John 3:4-6, where sin is the transgression of God's law, those who continue to practice seen have neither seen nor known him, and those who do nor practice righteousness in obedience to God's law are not sons of God. Again, Jesus being the living embodiment of God's law is what it means for him to be the Son of God, so he is the object of the faith that is expresses through obeying God's law, so Galatians 3:24-25 is speaking about before he came. If you agree that Christ is the only way to the Father and that what is listed in Hebrews 11 is example of people in the OT who had saving faith, then it follows that they therefore had faith in Christ, and they expressed that faith by obeying God, but the object of their faith had not yet come.