How to accurately detect all faces below a plane’s surface (not just at its center point)?

Exactly, imho if there are no points overlaying with the faces then the way could be compare target vs object all 2d flattened edge intersections. And then accumulate these edges on Face ID. I think it’s possible but far more complicated to setup. (Kind similar to Multiple curve intersections) In my answer I am simulating this by raycasting alongside edges but its still not 100% solution)