The Mosaic covenant was conditional. The Promise to Abraham was unconditional.
Agree. And Christ received those promises as Abraham's seed (as in singular.) Those who are Christ's also receive the promise as Abraham's seeds; and Paul was adamant that the heirs were both Jews and Gentiles:
"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." (Gal 3:16 KJV)
"For ye are all the children of God by faith in Christ Jesus. For as many of you as have been baptized into Christ have put on Christ. 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. And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise." (Gal 3:26-29 KJV)
Philip