While in deck or in discardpile, a lv.X is not a basic nor an evolution card. When in play, it is considered to be the same thing as the pokemon it leveled up from. So you can play a DRE on a lv.X.
Empoleon lv.Xs attack says, "does 80 damage". As long as it says damage, not "put damage counters" it is redused by 10, if Empoleon lv.X has a DRE attached to it.
Benching a lv.X will not take out the lv.X card from the pokemon.