Using Shock and Aftershock after having used earth overload actually removes the 3 second immobilize condition on the enemy player on the Shock and then reapply 2 seconds on the Aftershock. It is impossible to stack the immobilize duration together. Also if an enemy player dodges the Aftershock then he got a free cleanse from the Shock.
Beside that Tempest doesn’t work well in many of its aspects but I’m glad the elite is finally getting looked at. The 5 second cooldown before overloading is way too long (3 would be enough), and I do not believe that you should be penalized if it gets interrupted.