If I used Alakazam ex's "Dimensional Hand" attack from the bench on the opponent's Defending Pokemon with Handheld Fan attached, would one of Alakazam ex's Energy cards be moved or not?
Even though Alakazam ex is on the Bench it is still considered "the Attacking Pokemon". So the opponent's Handheld Fan would move an Energy Card from Alakazam ex to one of your other Benched Pokemon if you are able to do so.
Dimensional HandHandheld Fan