Agree to disagree but I don’t think either of your words capture “the whole argument hinges on only one specific point. If this point is refuted then the whole argument is refuted. The rest of the points are only auxiliary but not crucial”
Also a great way to disagree collaboratively. If you can collaboratively find the crux of each others' arguments, you now know both know exactly what would change each others' minds, and you can find that information together and know what's correct.