I need to remind you that Abraham was not a Jew, but was a gentile. He had nothing to do with the Mosaic Law. That came 4 centuries later.
Israel is Israel. It is the land of the tribes of Israel. The laws of Moses were for that nation and all in their households, even strangers of the Commonwealth of Israel. As long as any of them adhere to the Law instead of Jesus their Messiah, they remain under that everlasting covenant. It's a matter of choice. Christians are under the New everlasting covenan, the one that saves souls.
As for Galatians 3:28-29, let's first go back to 3:16...with my own commentary in parentheses...
Galatians 3:16-29 (KJV)
[SUP]16 [/SUP] Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
[SUP]17 [/SUP] And this I say, that the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect.
(because it came after the promise)
[SUP]18 [/SUP] For if the inheritance be of the law, it is no more of promise: but God gave it to Abraham by promise.
(the promise came first, fulfilled in Christ after the Law)
[SUP]19 [/SUP] Wherefore then serveth the law? It was added because of transgressions, till the seed should come to whom the promise was made; and it was ordained by angels in the hand of a mediator.
(Jesus came)
[SUP]20 [/SUP] Now a mediator is not a mediator of one, but God is one.
[SUP]21 [/SUP] Is the law then against the promises of God? God forbid: for if there had been a law given which could have given life, verily righteousness should have been by the law.
(hence, the law couldn't do it, but Jesus, the substance of the Promise, did)
[SUP]22 [/SUP] But the scripture hath concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe.
(not just them under the Law of Moses, including all the gentiles)
[SUP]23 [/SUP] But before faith came, we were kept under the law, shut up unto the faith which should afterwards be revealed.
(what else was there? The promise awaited Jesus to come)
[SUP]24 [/SUP] Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith.
(There's the end purpose, the law fulfilled in Christ)
[SUP]25 [/SUP] But after that faith is come, we are no longer under a schoolmaster.
(lack faith? Then ye yet remain under the Law which cannot save)
[SUP]26 [/SUP] For ye are all the children of God by faith in Christ Jesus.
(All Jews and gentiles who believe)
[SUP]27 [/SUP] For as many of you as have been baptized into Christ have put on Christ.
(that's the baptism Jesus came to give, not preaching John's baptism)
[SUP]28 [/SUP] There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus.
(assuming all conform to trusting the Lord and not the Law for salvation)
[SUP]29 [/SUP] And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
(not becoming a Jew, or being of Israel, but being of that promise made to Jesus, that promise shared by Abraham)
The Promise skipped over Israel because Israel rejected the seed of Abraham. The Promise skipped over the Law. The prophets warned them it was happening, and it did.
There is no saving virtue in leading anyone back to that which God put aside. It is now all Jesus Christ, or nothing.