Were the "all" just two categories as Calvin has it, it would already be fulfilled by the current "remnant" of Israel and Gentile believers. There would be no need for a further "mystery" moving it to a broader scale of fulfillment.
I agree. There were believing Jews, unbelieving Jews and gentiles. It wasn't Jews & Gentiles as Calvin thought. It was "Jew" vs. Jew as Jesus taught in Rev. 2:9 and 3:9. There are no Gentiles spoken of in Rom. 9: 6. Rom. 3:3,4 confirms that the "Unbelief" of the majority of Jews does not undermine the promises made to the remnant who believe.
Notwithstanding, there is zero hope for unbelieving Jews as well as unbelieving gentiles. The ' "all" Israel shall be saved' refers to the remnant of Jews, who at the Lords coming constitute the entire nation of Israel..... the unbelieving Jews having all been killed by the antichrist and his forces.
Leaping from there to a "Universal Horizon" is not supported unless you have something else.