This chapter is remarkable for its clarity about a most challenging topic. Apparently, Paul did not want there to be any question about what he thought about God's freedom to choose whomever He wants.
First, I notice Paul is in significant pain about this doctrine and particularly the lostness of his countrymen. This is not a smug theological treatise, "I've got God all figured out." He's grieved because not all Israel belonged to true Israel. And, this is his explanation for how that happens.
His singular justification is that God has willed it to be this way, so that salvation might be by mercy alone. He hardened Pharoah so that his name might be told throughout the whole earth. God wants his reputation and mercy to be spoken of throughout the world.
The choice of Rebecka's children was specifically made before they'd done good or bad so no one could construe they'd done anything to merit their salvation. That's the point. God didn't choose them because. . . of anything!