If an instant or sorcery card in your graveyard has {X} in its mana cost, X is 0 for the purpose of determining its mana value.

If you cast a spell “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the spell has any mandatory additional costs, such as that of Embrace Oblivion, those must be paid to cast the spell.

If the spell you cast has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you don’t want to cast the copy, you can choose not to; the copy ceases to exist the next time state-based actions are checked.

You cast the copy while Roving Actuator’s ability is resolving and still on the stack. You can’t wait to cast it later in the turn.