I’ve only just encountered your site, on which there are some nice things to think about!
“the Law would have stayed in force”
You seem to me to be implying that to be part of God’s people in the Old Testament, it was necessary to both have faith in God and to be under the Law. And then, that if there had been large scale following of (faith in) Jesus when he went about teaching, that that ‘faith and Law’ would have remained normative for God’s people, so that Gentiles would have had to also have ‘faith and Law’ to be part of God’s people. (Perhaps Jesus would not go through crucifixion if the latter had happened.)
If so, that would be ‘very puzzling’ things to think about!