Does Explosion/Self-Destruct 1/2 the opponent's Defense just for that Individual Attack or the remainder of the turn? Additionally, Does it divide the original Defense into 1/4 if both members of your team use it?
This has relevance for 2v2 games where the opponent survived the Explosion/Self Destruct.
Nope. If I remember correctly, Explosion simply halves defense to calculate the damage done by the Explosion. The actual defense of the pokemon doesn't change.