Returning "Iron Core of Koa'ki Meiru" from your hand to the top of the Deck is a cost.
If "Koa'ki Meiru Hydro Barrier" is removed from the field or flipped face-down after its effect is activated, the effect(s) of face-up Effect Monsters are still negated.
If your opponent Summons a monster while the effect of "Koa'ki Meiru Hydro Barrier" is being applied, that monster's effect will be negated.