Question

If angular velocity is zero than angular acceleration is

Answer 1

in your case the object is rotating with constant speed so tangential acceleration is zero so angular acceleration is zero. Angular velocity and angular acceleration are both in a direction parallel to the axis of rotation.

When the object reaches its highest point, then its velocity is 0 for a particular instant when it changes its direction but the object is still accelerating at a constant g=9.8 m/s.

Answer 2

Yes,as angular acceleration is the time rate of change of angular velocity.