There is probably a lot of use cases for many monitors that don't really require Windows (perhaps pro power-users, like traders, who need lots of browser windows open at once), so they need to ensure that their premium cards retain their edge even on Linux.
That makes much more sense.
That's a good point; I cannot say. Perhaps they think they are dealing with people who think 3D performance is necessary, or dealing with people (IT organizations?) that prefer 1st party drivers to hardware savings.
I don't think Microsoft is really involved in this though.
People buy hardware to use their advertised features. If you are the type of person that only needs 2D rendering and video then you should use an Intel GPU. I would assume people who buy a state-of-the-art dedicated GPU want to use them for actual 3D acceleration.