1) The ball is decelerating because of "damping". The default global damping is set in the Project Settings under Physics -> 2d -> Default Linear Damp. You can either set this to 0 or set your ball to override this with 0 in the Inspector.
2) The ball pushes the paddle because they are both RigidBody2D. Rigid bodies behave like physical objects, so when the ball hits the paddle there is a reaction. One workaround would be to change the mass of the paddle so it's much higher than the ball. Another common option is to use a KinematicBody2D for the paddle and move it manually along the x-axis when detecting input.