Some in-depth discussion of Reformed, Covenant Theology and Israel/Church identity over at James’ blog.
The best way I’m able to reconcile it in my head, as someone from a legal background, is to think of the arrangement as a third-party beneficiary relationship. The New Covenant was made with Israel, but Gentiles can–and eventually the whole world will–become beneficiaries of this “contract”. The get all the benefits of this renewed arrangement with Israel without having to perform the specific terms of the contract (i.e. the Torah mitzvot) as Israel has to. This may be a good way of thinking of the Church/Israel relationship, even if somewhat imprecise. Christians have no problem saying “Jesus paid it all”, but it may be helpful to conceptualize it as Yeshua as Israel, or Israel in the person of Yeshua making the payment.