I'm not entirely sure on questions 3, 4 and 6, that's why I rather not answer them.
So take these with a grain of salt (answer might be wrong):
I didn't find any mention of separate blending. My guess is you mean multi-pass blending? If you do, it means to build the final color of a pixel by several draw calls after each other.This lets you combine more than two sources for a final image.
Not really as far as I can see. You could however inject your custom factor by changing the vertex alpha or color by using D3DBLEND_SRCCOLOR or D3DBLEND_SRCALPHA. A different way could be changing the material attributes.
6) Not directly as far as I can see. This is a prime example for multi-pass blending though.
Ok. I see.
Thank you so much!