First of all there are 8 Covenants in the Bible;
(1) Edenic (2) Adamic (3) Noahic (4) Abrahamic (5) Land (6) Mosaic (7) Davidic
1-3, and 7 are all Covenants of Works (Law)
4 was a Covenant of Grace to Abram, accompanied by the promise of land.
7 was a Covenant of Grace to David, accompanied by the promise of an everlasting kindom.
So there are only Two covenants. The original Covenant of Works/Law and the New Covenant of Grace in Jesus Christ (that remedied the failure of the first).
Once Yeshua came He rendered the entire Mosaic Covenant inoperative as the rule of life for the believers.
Incorrect. The Decalogue is the eternal holy standard of God. Only through the New Covenant of Grace are sinners enabled to live pleasing to God, through faith in Jesus Christ.
All sinners are held morally responsible and accountable to God's holy standards under the demands of the Covenant of Works. All sinners are unable to keep this Covenant; therefore it condemns all.
The only remedy for this dilemna is the satisfactory fulfillment of the Old Covenant of works; achieved by the Son of Man, Jesus, on behalf of those souls He represented in His incarnate state. Because He fulfilled and satisfied all the legal and moral demands of the Old Covenant of Works, was a New Covenant of Grace established for the salvation of those who are justified through faith in His righteousness.
Going on inaccurately about the Covenants is a foolish endeavor, for when the information is wrong, the Gospel of Jesus Christ is skewed and becomes powerless to save.
Dispensationalists would do well to do a closer study of Covenant Theology.