Welcome to the Cake Decorators Q&A
About hand blender
Please advise is it mandatory to use hand blender when making the mirror glaze, or electric mixer can be used as well.
It isn't mandatory. You could use an electric mixer on a low setting to prevent too many bubble forming or just carry on mixing with a spatchula until everything is fully melted. The hand blender is good because it mixes everything up together without incorporating too much air into the glaze and ensures there is no clumping of ingredients. If you find your mixture has too many bubbles, pour it through a sieve and let it settle down before pouring.