If Radiant Jirachi with the "Entrusted Wishes" Ability is KO'd by Quaquaval ex's "Exciting Dance" attack which pushes it to the Bench after doing damage, do you still get to search your deck for up to 3 cards or not since Radiant Jirachi is on the bench by the time KO's are checked?
Yes. Entrusted Wishes checks for two things here: Radiant Jirachi was damaged by an attack while it was your Active Pokemon, and Radiant Jirachi was Knocked Out by that attack. So even though you don't check for a KO until the attack is completed, the criteria for using Entrusted Wishes has been met, so it can be used. This holds true for any similar Ability that triggers when the Pokemon is Active and gets damaged by an attack.
Exciting DanceEntrusted Wishes