Was wondering if the cracked shell debuff would make CL hit harder if targeting Horridon onto the adds. Haven't had the opportunity to try but my guess would be since the boss is the only with the debuff it might not.
IIRC correctly it didn't work on Windlord, so don't see it working here either. The increased damage taken should be applied after all the Chain Lightning math is done. Even if this wasn't the case you would have a very hard time if you were going to try to CL from horridon to the adds, they have to be directly under him.