In answer to your questions,
1) Yes that will work (provided your psu is up to the task, you don't mention its specs so we can't say)
2) Sorry, don't know much about video editing so I'm unsure whether or not it benefits from sli or crossfire. But that mobo supports both nvidia and radeon as well as upto quad sli or quad crossfire.
3) You heard wrong. Crossfire works just fine in windows.
4) No, it doesn't have to be all one or the other. You can run an intel cpu quite happily with an amd gpu.
2: The AMD Radeon HD series and the Nvidia GTX are the mainstream graphics cards on the market, meant for gaming performance. The Nvidia Quadro and AMD Firepro are dedicated rendering cards, meant only for video rendering.
If you stick to the mainstream cards, the Nvidia cards are better since you can utilize CUDA to assist in video rendering. Though if applications you use benefit from OpenCL, AMD cards are better at that.
I am also not sure if having multiple GPU's will help in video rendering, I would assume so since its double the hardware. Might wanna check on the AMD/Nvidia websites.