I'm pretty sure that OpenGL-1.0 is a typo. DirectX-11 is about the function level you get with OpenGL-4.0. Now look what key is right below the 4 on the numpad.
In fact no single NVidia GPU ever did support only a OpenGL profile as low as OpenGL-1.0. OpenGL-1.0 dates back 20 years.
Is CG now a viable alternative to replace GLSL 4.x?
Well, I personally don't see a reason why to use Cg, except if you want to support both OpenGL and DirectX with a common set of shaders. But why would you want cross API compatibility? If you aim for portability then OpenGL wins clearly over DirectX.
IMHO the main reason to keep using Cg is, if you have to maintain a legacy product that uses Cg already. Remember that Cg was introduced long before OpenGL had a high level shading language.
Can I write all shaders in CG that I could write in GLSL 4.3?
Yes.