If I copy Swanna's "Feather Dance" attack to add 40 damage to each of this Pokemon's attacks for example with Mew's "Lost Link" Poke-BODY, and then the following turn it copies an attack like Tyranitar's "Darkness Howl" that does 20 damage to each Pokemon in play, would the +40 damage be applied to Benched Pokemon or only to the Active Pokemon?
Swanna's Feather Dance applies to all damage done. So the attack would do 60 damage to each player's non-Darkness Pokemon, but you ignore Weakness & Resistance on the Benched Pokemon.
Feather Dance