Other known variants
+10AmnesiaChoose 1 of the Defending Pokémon's attacks. That Pokémon can't use that attack during your opponent's next turn.
Gastro AcidFlip a coin. If heads, the Defending Pokémon is now Paralyzed.
Almost all its body is its stomach. Its harsh digestive juices quickly dissolve anything it swallows.
If my opponent has Celebi-EX with "Time Recall" in play, can I use Golduck's "Amnesia" attack to prevent the Defending Pokemon from using one of its previous evolution's attacks?
No, you can only use Amnesia on the attacks that are present on the opponent's card at the time of your attack.
If Wooper uses Amnesia on Scyther's Slash, then Scyther retreats for another Pokémon, then that Pokémon [is Switched] for Scyther again, can Scyther use Slash?
Amnesia references the defending Pokemon, when that Pokemon is benched the effect ends.
My active Arbok uses its "Gastro Acid" attack against Weezing with the "Aftermath" Ability. Then on my opponent's turn he uses Weezing's "Smogbank" attack to do 20 damage to each of my Pokemon. If I have Latias-EX with the "Bright Down" Ability on my bench, would it take damage or not?
Since Weezing has no Abilities due to Arbok's attack, it would do 20 damage to Latias-EX.
Arbok's "Gastro Acid" attack says the Defending Pokemon has no Abilities until end of your next turn. Does retreating get rid of the effect so you can use Abilities?
Yes, it does.
Arbok's "Gastro Acid" attack says the Defending Pokemon has no Abilities until end of your next turn. Is that referring to the opponent's next turn or the attacking player's next turn?
It refers to the end of the next turn of the player who used Arbok's attack.