Playing a legendary land won’t cause the first ability to trigger.

The first ability of Rona, Herald of Invasion will resolve before the spell that caused it to trigger.

The spell you cast from exile without paying its mana cost is cast during the resolution of the triggered ability. Timing restrictions of that spell based on card type are ignored.

If the spell 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 cast a spell without paying its mana cost, you can’t choose to cast it for any alternative costs. You can, however, pay any additional costs. If the spell has any mandatory additional costs, you must pay those.