1. can you make a video do a quick blur then go back to normal, like in action movies?
Yes, you can by applying effects like a directional blur and animating their parameters.
2. i already know how to make the green screen transparent, but how do you add your own background?
You simply put another layer below it. No offense, I feel that even the most basic concepts of AE elude you, so I strongly suggest you read this and work your way through. No point in getting strung up on specific advanced tutorials and then having no clue how to get the rest done.