Agreed about Psalm 22. Whatever may be about the hands and feet (and MT disagrees with LXX here) the psalm is in no way describing crucifixion, far less the death of a messiah on a cross.If I understand correctly your point is: Paul uses crucifixion of the Lord against Jewish Law, because it is written “for a hanged man is cursed by God" (Deuteronomy 21, 23). Let us forgot that this sentence refers to a corpse and not a living man in a cross. Let us also forgot that Psalm 22 sentence doesn't clearly refer to nailed hands and feet. The fact is Paul believed so. And your argument is that Paul could invent the entire cross story as a weapon against the Law of Judaism, i.e., as a proof that Law was abolished with the death of Jesus.
There is another issue about the interpretation of Deut 21:23. See http://mailstar.net/maccoby.html. Hyam Maccoby claims that Paul's interpretation of the verses was not standard in Judaism.
I see no good reason to suppose that the crucifixion of Jesus is invented by Paul. He reinterpreted it, in terms of his understanding of the Law, which may not have been accurate on this point.It is clear that the manner of Jesus's death, by hanging on a cross, was itself of great significance to Paul ... Here Paul has recourse to an interesting interpretation, or misinterpretation, of a verse in the Hebrew Bible (which Paul, incidentally, read in Greek, not in Hebrew, since it can be shown that his quotations are from the Septuagint ... Deuteronomy 21:22-3 reads (in the Authorized Version):In Jewish exegesis, this was not held to mean that a hanged man was under a curse, for, on the contrary, it was held that an executed man was purged of all guilt by his execution. If his dead body, contrary to the law, was allowed to hang overnight, this could bring no curse upon the dead man, who had paid the penalty of his crime, but only upon those who contravened the law by exposing his body. Thus the relevant sentence is translated in the New English Bible, 'A hanged man is offensive in the sight of God', a translation very much in accord with Jewish traditional exegesis. Paul, however, understood the sentence very differently. His comment is, 'Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree' (Galatians 3:13).And if a man have committed a sin worthy of death, and he be to be put to death, and thou hang him on a tree: his body shall not remain all night upon the tree, but thou shalt in any wise bury him that day; for he that is hanged is accursed of God; that thy land be not defiled, which the Lord thy God giveth thee for an inheritance.