For example, does Arceus-Bug get the 20% power boost for his Bug type attacks?
Yes, elemental plates boost the power of moves of the same type. So if Arceus bug used a bug type move it would be powered up.
Source: My knowledge.
Yes.
However, it will also increase the attack power of moves of the Plate's type used by any Pokémon holding it by 20%.
http://bulbapedia.bulbagarden.net/wiki/Plate Source: Experience and clarified by Bulbapedia.
