Maybe this would help. There is one program/plan of redemption that is unified at all points. However, within this program, there are two tracts. One for Israel, and one for the church. And by Israel, for those who keep not doing this, you have to understand that this is the Nation of Israel, of which there are those who are truly of Israel, and those who are not. The remnant, the spiritual Israel of God's elect Jews, and the Nation of Israel, the secular, those who are not of the remnant, the elect.The first chapter of Ryrie's book, a text at Dallas Seminary for decades, begins "The distinction between the two programs--Israel and the church--is the sine qua non of Dispensationalism." (s.q.n. = the single defining thing).
You really do not know what Ephesians is saying. All your allusions are from Romans. The main thing is that the mystery was not that Gentiles would believe, it is that that they gain co-inheritance and co-membership status with "Israel" through the Gospel, 3:6. That is exactly where the member of Judaism would have expected to read 'through the torah.'
Yes, a remnant is always saved, but as Isaiah said "It is too small a thing for (the Servant) to save the remnant of Israel; (God) will make (the Servant) a light to the nations." Proof again that the mission to the Gentiles is what Israel was destined to do, and was no surprise or mystery.
Redemption was ALWAYS to come from Israel, and even David knew that. The Messiah would be of Israel, of David's seed. When they rejected, as Paul says, the gospel went to the Gentiles. And at the end, it will come back to Israel. You have to stop burying what Paul said. There is a blindness, but Paul specifically stated that it will be lifted when the times of the Gentiles has come in. At this point, God will deal with the Nation of Israel, after which, the only survivors will be the elect/remnant of Israel, and they will be saved. And again, there are Jews in the church, and they will continue to join, but the bulk have been blinded until the fullness of the Gentiles has come in. And again, we are ALL SAVED BY THE SAME THING, IN THE SAME MANNER. It is one plan of redemption. Just because there are two different groups that take two different ways to get to the same conclusion, doesn't mean there are two conclusions.