Maybe someone else can help out better than I but it seems the first problem is that you are not creating a solid just a 2D object and I think the spline modeler is having difficulty with that. Rather than just putting a line on the cross section why not a very thin box. Additionally there should be no need to make any other cross sections to make a ribbon with variations in width as you show here. You will also find that handles on curve points can be adjusted in ALL plains kind of independently. Hope this puts you on target....