You must follow the normal timing permissions and restrictions of the cards you cast from exile.

Once you begin to cast a card, losing control of Rona won't affect the spell.

The card you'll exile from the top of your library is determined as Rona's last ability resolves. This might not be the card that was on top of your library when you activated that ability.

The cards exiled from your library are exiled face up.

After Rona's last ability resolves, you have priority if it's your turn. You can cast the exiled card before any player can take other actions if it's legal to do so.

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

The exiled cards aren't in your hand, so you can't cycle them, discard them, or activate any of their activated abilities.

A card, spell, or permanent is historic if it has the legendary supertype, the artifact card type, or the Saga subtype. Having two of those qualities doesn't make an object more historic than another or provide an additional bonus—an object either is historic or it isn't.

You'll still pay all costs for that spell, including additional costs. You may also pay alternative costs such as that granted by Jodah, Archmage Eternal.