You're making it way too complicated. The power-line company can buy power from whoever is providing, and sell power to whoever is consuming. Just like they do now with home solar power. They can make whatever agreements they like with generating companies as to who gets what share of demand, what response times are required, etc. Add some grid-scale power buffers, even just a few minutes worth, and things get even simpler.
While it sounds good in theory, power plants and grids take years to expand, and the information needed to project power plant demand would be in the hands of the grid owners. It would be much harder to scale power generation smoothly with demand, and potentially increase the number of power company crashes due to the potential boom/bust introduction. This would allow the line owners to control the winners and losers in the power generation industry, and create a scenario similar to the way content producers like Netflix interact with Comcast.
While the potential for competition would be nice, the potential for boom/bust crashes of power generating companies could create instability and brownouts, something many deem unacceptable. In this case, I think industry stability > industry growth.