"Are we heirs of the first covenant God made with his chosen people, the Israelites?"
Christologically speaking we do become heirs of that promise/covenant through the Sacraments.
I get the impression that there is only one covenant that God adds layers to over time until His coming as the Person of Christ.
First we had the covenant made between Adam and Eve that Christ will come as man (Genesis 3:14-15). Then God changed Abram's name and added the sign of circumcision to seal the promise that Christ would be a descendant from Abraham/Israel (Genesis 17:1-14).
Then comes the adding of blood sacrifice for forgiveness of sins to the covenant (Exodus 24:1-8).
Then, finally, Jesus replaces circumcision with Baptism and blood sacrifice with "the new covenant in My Blood", being Holy Communion. (It's possible an argument could be made that with His coming the OT covenant is fulfilled and that Jesus intended the Word & Sacrament to be an entirely new covenant for "all nations" separate from the OT version but still related to it in a traditional sense. Although I am not convinced this would be a right interpretation based on Romans 11--specifically verses 17-18 and 25.)
The point is that God's grace through Christ is a universal Gospel; the promise of salvation for all people in Scripture--both testaments [Isaiah 53:5-6,56:7-8; Psalm 117:1-2; Matthew 28:16-20; Luke 2:29-32; Romans 11]--comes not from an exclusionary God, but from an inclusive God .