A loft needs the spline used for the path to be linear. You can´t use a complex spline with branching in it for a loft. But you could make your spline renderable and give it a specific thickness. select spline > go to modify panel, under 'Rendering section' > check "Enable in Renderer", check "Enable in Viewport" if you like to see actual thickness in viewport. "Thickness" can adjust line's thickness. Interpolation is an extra that can help you smooth the line shape. Maybe this could help you out.