Jeleva, Nephalia's Scourge

If Jeleva enters the battlefield without being cast, then no mana was spent to cast it. This means X will be 0, so no cards will be exiled.

You can't choose to pay extra mana to cast a creature spell unless something instructs you to.

The amount of mana you spent to cast this creature is usually equal to its mana value. However, you also include any additional costs you pay, including the cost imposed for casting your commander from the command zone.

If Jeleva leaves the battlefield, the exiled cards will remain exiled indefinitely. If Jeleva enters the battlefield again, it won't be associated with the cards the "other" Jeleva exiled. The new Jeleva will exile a new set of cards. Only those cards can be cast using Jeleva's last ability.

When casting an instant or sorcery card this way, ignore all timing restrictions based on the card's type. Other timing restrictions, such as "Cast [this spell] only during combat," must be followed.

If you cast an instant or sorcery card using Jeleva's last ability, you do so while that ability is resolving. If you choose not to (or you can't), you won't get a chance to cast it later unless the ability triggers again.