Once you begin casting a spell from your graveyard, it immediately moves to the stack. Players can't take any other actions until you've finished casting the spell.

If Rona is countered or dies after being cast from your graveyard, it returns to the graveyard. It may be cast this way again later.

Rona, Sheoldred's Faithful's triggered ability resolves before the spell that caused it to trigger. The ability will resolve even if that spell is countered.

The casting permission granted by Rona's last ability doesn't change when you may cast it from your graveyard.