I'm sorry, but...what? Absoultely NONE of that makes a lick of sense. At all. Any way you look at it. From a covenantal point of view, or just from a rational point of view.
How can something be fulfilled by Christ, but not ultimately be about Christ? The verse says "TO"...Christ.
How can a covenant with Abrahams decendents be based on Abrahams covenant. A covenant by itself is dead. A covenant has to be between two parties, not another covenant. A covenant with Abrahams 'decendants' has to be with the Covenant maker...or, it has to be part of the Abrahamic covenant. Otherwise it's a promise resting on a promise. And promises, without the people making them, are dead. They're just empty words.
Also...your link to "who are the seed of Abraham"?....it sort of supports my view, just fyi. That Gal 3:16 says that the promise is made to Christ, and that those who are "in" Christ are also heirs to the promise. Good link though...
Besides, you are clearly missing, or dodging, the clear wording of this verse:
Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings,” referring to many, but referring to one, “And to your offspring,” who is Christ. -Galatians 3:16
It outright says that the "promises were made to Christ". See how that flows? The promises were made to Abraham AND TO his offspring. Those 'offspring' actually being one...Christ. Thus, the promises were made to Abraham AND TO CHRIST. Not fulfilled BY Christ, but made TO him.
Then, you quote this verse (kudos for actually using a bible verse, by the way):
There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus. And if you are Christ's, then you are Abraham's offspring, heirs according to promise. -Galatians 3:28–29
However, I don't see how it helps your argument at all. Paul is saying that IN CHRIST there is no Jew or Gentile. There is only one people and we are Abrahams offspring, heirs to the promise. In regards to the link you posted, we are, IN CHRIST...'spiritual Israel'.
And when we look at this verse in conjunction to:
But if some of the branches were broken off, and you, although a wild olive shoot, were grafted in among the others and now share in the nourishing root of the olive tree, do not be arrogant toward the branches. If you are, remember it is not you who support the root, but the root that supports you. Then you will say, “Branches were broken off so that I might be grafted in.” That is true. They were broken off because of their unbelief, but you stand fast through faith. So do not become proud, but fear. For if God did not spare the natural branches, neither will he spare you. -Romans 11:17–21
Then we can see that not all "physical Israel" remain on the 'tree'. They are no longer 'heirs to the promise'...only those who are "In Christ" are heirs to the promise.
But...this is not "replacement theology", where the Church has kicked Israel out of her place. No...Israel stands in the place she always has...true spiritual Israel, Abraham, Isaac, Jacob, David...those elected by grace and faith in the OT who 'stood firm'...they were true Israel. And once Christ came, any of the nation who rejected him, blood or not, was 'broken off', and those who accepted him, blood or not, were 'grafted in'. This is not 'replacement theology' it is expansion theology, or adoption theology. And a good portion of us believe that Romans goes on to say:
And even they, if they do not continue in their unbelief, will be grafted in, for God has the power to graft them in again. For if you were cut from what is by nature a wild olive tree, and grafted, contrary to nature, into a cultivated olive tree, how much more will these, the natural branches, be grafted back into their own olive tree.
Lest you be wise in your own sight, I do not want you to be unaware of this mystery, brothers: a partial hardening has come upon Israel, until the fullness of the Gentiles has come in. And in this way all Israel will be saved, as it is written,
“The Deliverer will come from Zion,
he will banish ungodliness from Jacob” -Romans 11:23–26
I fully believe God still has a plan for 'physical' Israel. But it is not as a separate people. It is as 'spiritual Israel', just as we are...all one in Christ...our Messiah.