Normal.cs

Vector3D Normal(Solid3D.Face face, Point3D p)
{
    return face
        .Parametric
        .SelectMany
        (s => s.PointInversion( p, 1e-3, out var coord2D ) 
             ? Some((p: s.PointAt(coord2D), n: s.Normal(coord2D))) 
             : None )
       .MaxBy(q => p.DistanceTo(q.p))
        [0].n;
}

Trick is that Optional implements IEnumerable so works with SelectMany to flatten it.