Are Christ-rejecting Jews following their conscience in that rejection?
Here's how it all works, in my understanding. Bear in mind that a person need not know about the historical, incarnate Jesus to be saved - the OT saints were saved without this information.
Every monotheistic person, including Jews and Jehovah Witnesses, worships "God." The problem is that the feeble human mind, on its own, cannot form a reasonably accurate picture of an ineffably holy God. As a result, the unaided human mind will always worship a conceptual idol.
To remedy this, God has always provided - to both OT and NT individuals - two acceptable pictures of Himself. Submitting to either of them is salvific.
....(1) General revelation (Romans 1:18-20). The human heart/conscience is designed to infer, from the beauty and grandeur of Nature, the existence and holy character of God. Admittedly this portrait of God is somewhat vague - but it is approved of God and therefore acceptable in His sight.
....(2) Special revelation. The inward witness of the Holy Spirit. This portrait is an enhancement of General Revelation - a sharpening of the image/picture already there. Special revelation typically includes extra details, for example it might convict/convince the individual that the bible is inspired, that Jesus is God, that He died for ours sins, and even God's triune nature.
Given our hardness of heart since the Fall, it's debatable whether anyone accepts General Revelation by itself. Most, if not all people, probably won't submit to a picture of the Lord until Special Revelation intervenes (e.g. Paul's vision on the Road to Damascus).
If a person DOES submit to General Revelation, I'm pretty sure that God, as a reward, immediately grants him Special Revelation. As a result, a Christ-rejecting Jew CANNOT be a true believer in the Lord, clearly he hasn't submitted to ANY valid picture of God.