Unfortunately, unlike earlier embedded video, the Intel HD video processor resides on the CPU die. Most (if not all) PCs using this form of integrated graphics (Intel Core i3, i5,i7 & AMD APUs) don't have the option to disable on CPU video... At least none that I have seen or worked on...
The GPU switching feature has only been recently introduced (in the past 3 or so years) and, AMD isn't known for their great driver-writing skill...
Now for the second part of your question...
I could be wrong, but graphics switching is most prevalent in the mobile field, where battery life is taken into consideration... The second GPU you listed (NVidia) is a desktop GPU, and AFAIK, Intel HD Video will not run in conjunction with a discrete NVidia GPU for the purpose of graphics switching...
NVidia did have a GPU switching technology for desktops using NVidia chipped boards and NVidia IGP and NB chips that allowed switching between the on-board IGP and Discrete GPU depending on graphics load. It wouldn't work if the integrated video were Intel or ATI. It was an extension of Hybrid-SLI/Optimus as far as I remember.
Amd also allows an integrated/discrete X-Fire in AMD chipped boards, but still not with Intel.
Keep in mind I have been out of the loop for a while, life has kicked me in the hind-end a few times in the past year and a half...
Perhaps there is a desktop variation of the graphics switching that I am unaware of, but my research keeps leading me to the portable market.
