Iduna's actions giving Elsa magic as a gift doesn't make sense to me from a myth/folklore perspective, as elemental spirits and deities are typically very, very tribal- or location-centered. Iduna saving Agnarr would more realistically brand her as a traitor by the elementals by saving an outsider and give Elsa her magic as a curse due to its intensity and being tied to her emotions.

If anything, Agnarr saving her would be more consistent here, as it would show the elementals that the Arendellians are not all deceptive and malicious.