x2
-20Severe PoisonYour opponent's Active Pokémon is now Poisoned. Put 4 damage counters instead of 1 on that Pokémon between turns.
Skill DiveChoose 1 of your opponent's Pokémon. This attack does 30 damage to that Pokémon. (Don't apply Weakness and Resistance for Benched Pokémon.)
If I had used Muk & Alolan Muk GX's "Severe Poison" attack that places extra Poison damage counters and in the next turn use Koga's Trap, do they still place the extra Poison damage?
No, they don't. The new, normal Poison Special Condition replaces the old Severe Poison Special Condition.
If I Switch my Active Pokemon that is affected by Poison from Muk & Alolan Muk GX's "Severe Poison" attack and Dust Island is in play, does my new Active Pokemon take the extra damage counters?
Yes, it does. Dust Island specifies that it is affected by "that Special Condition".
If my opponent's Florges used its "Moonblast" attack last turn, would my Kilowattrel's "Skill Dive" attack only do 20 damage to a Benched Pokemon?
Yes, all attack damage is reduced by 30. This damage reduction applies to all Defending Pokemon (including Benched Pokemon), and also applies to any self-damage done to your own Pokemon.