Fresnel is dependent on the microfacets oriented towards the halfway vector, but diffuse actually is all the light that is not being reflected, not being absorbed and scattered back out, independent of the microfacets oriented towards the halfway vector. So simply the complement of a single fresnel value won't do it. You would need to solve the integral over all the microfacet orientations with a modified microfacet model:
Your approximation might actually be worse than not having a factor for diffuse at all. If anything I'd use the macro surface normal instead of the halfway vector (just for diffuse though, for specular you should use the microfacet normal):
