Lileep

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.