Why would Paul forget to mention "of law" to Gentiles in vv. 8-10, if it were so all-fire obsessive an issue of his?
There is no reason to connect v. 15 with v. 8-10. There's really nothing connecting them. Paul doesn't talk about "works" around v. 15; Paul doesn't talk about "law" around v. 8-10.
Implying "of law" on top of everywhere Paul talks simply about "works" doesn't work, anyway. Paul's argument is about works, not a narrow works of law. That's demonstrable from Romans 4:1-5, which is pre-law.
Mike, it's a letter. You've got to remember you're reading one side of a conversation. We don't know what questions if any the Ephesians asked Paul. His letter may be in reply to a letter from Them (which I think is likely). The fact that he speaks boasting in verse 9 shows that he has the works of the Mosaic Law in mind. Not to mention that he goes on to explain why it's not of works.
One thing many Christians don't think about with this works issue is this. The Jews were promised an inhabitance in the promised land if they obeyed God word. The Jewish mindset was if I obey I can earn an inhabitance in the promised land. To the Jews, it was obey, obey, obey. By being obedient to the Mosaic they could win favor with God. Now here comes Paul preaching to the Gentiles telling them they don't need to be obedient to the Mosaic Law. This would turn the Jews world upside down. This is one of the reasons Paul was rejected by the Jews.
26 Then Paul took the men, and the next day purifying himself with them entered into the temple, to signify the accomplishment of the days of purification, until that an offering should be offered for every one of them.
27 And when the seven days were almost ended, the Jews which were of Asia, when they saw him in the temple, stirred up all the people, and laid hands on him,
28 Crying out,
Men of Israel, help:
This is the man, that teacheth all men every where against the people, and the law, and this place: and further brought Greeks also into the temple, and hath polluted this holy place. (Act 21:26-28 KJV)
It was well known that Paul taught that the Law was not necessary, as is evident from the passage it was not popular among many Jews. This is the background in Paul is writing. As I pointed out in the last post, there were Jews teaching his converts that faith in Christ alone was not enough. Many Jewish believers kept the Law even after being saved and as was seen in acts some in the church at Jerusalem even believed it was necessary for the Gentiles to keep the Law. Paul probably ran into this problem most places he went.
Even looking at Romans 4 which you posted. Here again the issue is the Mosaic Law. Look at chapter 3, Paul is making the argument that a man is not justified by the works of the Law.
19 Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God.
20 Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law
is the knowledge of sin.
21 But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets;
22 Even the righteousness of God
which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference:
23 For all have sinned, and come short of the glory of God;
24 Being justified freely by his grace through the redemption that is in Christ Jesus:
25 Whom God hath set forth
to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;
26 To declare,
I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.
27 Where
is boasting then? It is excluded. By what law? of works? Nay: but by the law of faith.
28 Therefore we conclude that a man is justified by faith without the deeds of the law.
29 Is he the God of the Jews only?
is he not also of the Gentiles? Yes, of the Gentiles also:
30 Seeing
it is one God, which shall justify the circumcision by faith, and uncircumcision through faith.
31 Do we then make void the law through faith? God forbid: yea, we establish the law. (Rom 3:19-31 KJV)
To prove his point that no man is justified by the works of the Law, he poinst to Abraham (chapter 4) who lived before the Law. What better way to prove that a man is not justified by the Law than to show a man who was justified before the Law existed? He concludes that Abraham was not justified by works (The works he's been talking about, the works of the Law)
KJV
Romans 4:1 What shall we say then that Abraham our father, as pertaining to the flesh, hath found?
2 For if Abraham were justified by works, he hath
whereof to glory; but not before God.
3 For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness.
4 Now to him that worketh is the reward not reckoned of grace, but of debt.
5 But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness.
6 Even as David also describeth the blessedness of the man, unto whom God imputeth righteousness without works,
7 Saying, Blessed
are they whose iniquities are forgiven, and whose sins are covered.
8 Blessed
is the man to whom the Lord will not impute sin.
9 Cometh this blessedness then upon the circumcision
only, or upon the uncircumcision also? for we say that faith was reckoned to Abraham for righteousness. (Rom 4:1-9 KJV)
Notice Paul says if it is of works it is reckoned as a reward. Think back to the Jewish understanding. If you obey me and do this, and this, and this, you can stay in the land, but, if you disobey you will be kicked out of the land. Paul is arguing that justification doesn't come through keeping the Mosaic Law, which would be earned because God said if you do this, I will do this. Rather it is by faith, and not the works of the Mosaic Law