"Abraham’s faith precedes the Law (and circumcision). So does the ‘new
creation’ idea take us back to the prior ‘justification’ of Abraham -
and does it therefore add weight to the view that Abraham’s family was
originally conceived as an alternative ‘creation’?"

I think it might be more accurate to say that, by believing God, Abraham received the promise of a new creation, a promise which was to be fulfilled in the future. This promise has now been fulfilled in Christ’s death, ushering in the new creation. All who believe God can receive now that which had been promised long ago to Abraham. This is the thrust of Paul’s argument in Galatians:

Abraham BELIEVED GOD, AND IT WAS RECKONED TO HIM AS RIGHTEOUSNESS. Therefore, be sure that it is those who are of faith who are sons of Abraham. The Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, saying, "ALL THE NATIONS WILL BE BLESSED IN YOU." So then those who are of faith are blessed with Abraham, the believer. (Gal. 3:6-9)

Throughout the centuries the Jews anticipated the fulfillment of God’s promise to Abraham, which remained in the future. They failed to recognize that participation in the Mosaic Law — i.e., membership in the nation of Israel — wasn’t the portal offering entry into the new creation yet to come. The real portal is faith, by which many nations may participate in the fulfillment of the promise.

Many nations are to be blessed by the fulfillment of God’s promise to Abraham. In Gal. 3:16 Paul emphasizes that "the promises were spoken to Abraham and to his seed": singular, not plural. Paul goes on to say that the seed of Abraham is Christ. I think Paul’s intention here is to assert that the promise wasn’t extended directly to the many nations that would eventually benefit from the promise, nor was it channeled through Israel the grandson of Abraham, nor through Moses by whom the Law was given to the nation of Israel. Instead, the promise passed through Jesus.

What I am saying is this: the Law, which came four hundred and thirty years later [i.e., after Abraham], does not invalidate a covenant previously ratified by God, so as to nullify the promise. For if the inheritance is based on law, it is no longer based on a promise; but God has granted it to Abraham by means of a promise. Why the Law then? It was added because of transgressions, having been ordained through angels by the agency of a mediator, until the seed would come to whom the promise had been made. (Gal. 3:17-19)

Paul is saying that the promise to Abraham, the "new creation" through which many nations would be blessed, remained a promise rather than a fulfillment throughout all the intervening centuries between Abraham and Christ. It wasn’t until now, through Christ’s death, that the promise is fulfilled and the new creation begins.

But before faith came, we [i.e., the Jews] were kept in custody under the law, being shut up to the faith which was later to be revealed. Therefore the Law has become our tutor to lead us to Christ, so that we may be justified by faith. But now that faith has come, we are no longer under a tutor. For you [i.e., the Gentile believers] are all sons of God through faith in Christ Jesus. For all of you who were baptized into Christ have clothed yourselves with Christ. There is neither Jew nor Greek, there is neither slave nor free man, there is neither male nor female; for you are all one in Christ Jesus. And if you belong to Christ, then you are Abraham’s descendants, heirs according to promise. (Gal. 3:23-29)

There remains, as you point out, an element of the promise yet to be fulfilled, an aspect of the new creation not yet made manifest.

For we through the Spirit, by faith, are waiting for the hope of righteousness. (Gal. 5:5)

Paul directs this remark to the believers who, realizing that they haven’t yet become sinless, are tempted to revert to the Law as the means of achieving sanctification. Paul is reassuring the believers that, even though the promise is now fulfilled in Christ and the new creation is begun, it’s not yet complete. And so Paul begins his discussion of walking freely in the Spirit rather than obeying the Law as the right way to live in new creation.