The direction of the electric field line is the direction of the force on the positive test charge. The acceleration is parallel to the net force by Newton’s second law, so yes, the acceleration direction is along the field line. The velocity cannot be determined this way.
Work Step by Step
Assume that there are no other forces acting on the test charge. The direction of the particle’s velocity is not necessarily along the field line. It could have been moving before it arrived at that point. All we know is that the field line indicates the direction of the acceleration, i.e., the direction of the change in velocity.