How did the law close the door to the Gentiles if they in fact could choose to follow it?
Yes, Alex, that’s probably overstated, though i) it makes a huge difference that the Law has been removed as a requirement for the Gentiles; and ii) it had come to the point for Israel that the Law pronounced judgment rather than blessing on the people of the Law—even those Gentiles who converted would presumably have come under the same judgment. As Jesus said: “Woe to you, scribes and Pharisees, hypocrites! For you travel across sea and land to make a single proselyte, and when he becomes a proselyte, you make him twice as much a child of hell as yourselves” (Matt. 23:15).
Rahab comes at a different point in the story, when Israel, for all its failings, was coming into the land. When he get to Jesus, Israel faces being decisively rejected from the land.
As for your second question, I think the point is that Jesus’ death and resurrection anticipate the whole story of his disciples’ continuation of his prophetic ministry towards Israel: his suffering foreshadows their suffering (including the turmoil of the run-up to war), and his resurrection foreshadows their eventual vindication. If the disciples had not bought into this narrative, and if others had not been baptized into it, there would have been no salvation for the people of God. The basic argument, therefore, that the only path by which the community of Jesus’ followers would find life was a path of Christ-like suffering in faith that God would see them through to life.
It’s late here, and this may not make complete sense. Let me know. I’ll come to your comment on the Romans post, which Ive just seen, tomorrow.