Elite Arcanist

You can't pay any alternative costs such as overload costs. You can pay additional costs such as kicker costs. If the spell has any mandatory additional costs, you must pay those.

You don't choose the value of X generic mana in the activation cost of Elite Arcanist's ability. It's defined by the mana value of the exiled instant card.

If Elite Arcanist leaves the battlefield, you will no longer be able to copy the exiled card. If it returns to the battlefield, you'll be able to exile another instant card from your hand, but Elite Arcanist will be a different permanent with no connection to the first exiled card.